On the acquisition of potential verbs and conjugation types of verbs in Japanese

5.1 Initial stage (around the age of 2 years)

With regard to the initial stage of the acquisition of potential verbs as stated in (16), Fuji et al. (2008a,b) take zero potential forms in (5a–c) as evidence against the availability of the head v. In the analysis proposed in Fuji et al. (2008a,b), for sim-e-ru, the morpheme e placed after the root sim is not regarded as the v head, but rather sim-e-ru as a whole occupies the V head. What is to be noted here is that sim-e-ru “close” and tabe-ru “eat” that appear in the relevant data are uniformly in the class of vowel-ending verbs. Thus, zero potential forms from consonant-ending verbs such as sim-ar-u “close,” the intransitive counterpart of sim-e-ru, for example, could emerge in children’s natural speech in principle as long as conjugation types are irrelevant to the formation of zero potentials. However, this prediction is actually not borne out: we found no cases in which consonant-ending verbs are used as zero potential forms as such in the corpus-based data available in Okubo (1967), Noji (1974–1977), Ito (1990), Shibuya (1994), Arai (2006), and Yano (2007). This clear contrast concerning the conjugational morphology between vowel-ending and consonant-ending verbs in the production of zero potential forms cannot be captured straightforwardly under the analysis by Fuji et al. (2008a,b) without any auxiliary assumptions or stipulations.

In fact, the correlation between the formation of potential forms and conjugation types of verbs is also confirmed in e potential verbs in (6a–c) and (7a–c), though in a way contrast to the situation of zero potential forms. First, until the age of 5 years when children establish the grammatical system to produce potential verbs, they consistently and more frequently utter potential verbs on the basis of consonant-ending verbs as shown in (6a–c), rather than vowel-ending verbs as shown in (7a–c). The relevant data are repeated here as (18), (19), and (20).

This fact could be regarded as a reflection from the frequency of language input after birth. That is, since there are a much greater number of consonant-ending verbs than that of vowel-ending verbs in input, children have learned more five-grade conjugation verbs at the early stage of language acquisition (Fujimura 2004). Given this situation, a simple question naturally arises as to why zero potential forms based on consonant-ending verbs are not produced frequently at all when children at this stage of language acquisition have learned more consonant-ending verbs than vowel-ending verbs. Since, as stated in (16), Fuji et al. (2008a,b) assume that children around the age of 2 years have yet to obtain the grammatical rule concerning the morphological realization of v and that accordingly zero potential forms and e potential verbs are taken uniformly as individually learned lexical verbs, such a correlation between the morphological class of verbs on the one hand and the formation of zero potential forms and e potential verbs on the other cannot be simply predicted.

As the starting point for our argument, let us go back to the two possibilities with regard to the production of e potential verbs in the initial stage as summarized in (8a,b), which are repeated here as (21a,b).

Fuji et al. (2008a,b) base their analysis on the position of (21b); hence, both zero potential forms and e potential verbs are lexical or idiomatic in the sense that their utterances are not viewed as being productive at all. While utterances of zero potential forms in natural speech data are not frequent at all, we still have good reasons to argue that e potential verbs uttered by children of this age are not separately learned lexical items but rather they are already being produced productively. Shibuya (1994) conducted an important research with respect to this issue to gather speech records from Infant Language Materials 1–6 (from the National Institute for Japanese Language and Linguistics. 1982–1983. Shuei Shuppan.) and examined the acquisition of potential verbs by tracking one boy’s utterances from the very beginning of his natural speech. Shibuya (1994), while being cautious about making generalizations, gives an interesting suggestion in (22) concerning the example of (20b), which contains e potential verb sim-e-re-ru “being able to close” based on the vowel-ending transitive sim-e-ru “close.”^[5]

Given that expressions such as sim-e-re-ru “being able to close” are not considered to be included in the language input by the mother who does not use ra-dropped expressions, the fact that Child T can express such ra-dropped expressions is not totally expected in the position of (21b), which denies any productivity in the formation of potential verbs by the child around the age of 2 years. In contrast, if we adopt the position of (21a) and assume that the child has some kind of internally designated grammatical mechanism responsible for producing potential verbs even in the very early stage of language acquisition, e potential verbs including noncanonical ra-dropped expressions will be easily predicted to occur in the child’s natural speech. The analysis by Fuji et al (2008a,b) on the position of (21b), therefore, cannot be valid empirically in that it does not make right predictions concerning the productivity in forming potential verbs by young children.^[6]

Now let us turn to the contrast between conjugation types concerning the formation of zero potential forms and e potential verbs discussed earlier. From the observation by Shibuya (1994), it is strongly suggested that children around the age of 2 years have already acquired grammatical mechanism to construct the structure of VP coupled with knowledge concerning the productivity in deriving potential verbs. The next question to be answered at this point is stated as in (23).

To answer these questions, let us remind ourselves of facts that the acquisition of causative/inchoative alternation that proceeds chronologically in a parallel manner to that of potential verbs and that erroneous uses in (in)transitivity are frequently observed around the age when children also begin vocalizing potential forms. The relevant data are repeated here as (24a,b).

As already mentioned earlier, Fuji et al. (2008a,b) treat these errors in (in)transitivity as proof of the lack of the productivity in forming potential verbs due to the unavailability of knowledge concerning the realization of the head v. However, previous studies dealing with such erroneous uses in causative/inchoative alternation, including Ito (1990), Morikawa (1997), Nomura and Shirai (1997), and Nakaishi (2016), have reported that the ratio of the occurrences of errors in causative/inchoative alternation in the period from the age of 1 year and 11 months to 3 years and 3 months by a single child is no more than 1.2% of the overall speech, which is much smaller than what is to be expected under the analysis by Fuji et al. (2008a,b). Moreover, according to the abovementioned previous research, it is difficult to identify the direction of errors: there are some transitive forms of verbs used for their intransitive counterparts and vice versa. In fact, a close look at the data reveals that those errors in causative/inchoative alternation in question never occur randomly; rather, as observed in the transitive todok-e-ru “deliver” from the intransitive counterpart todok-u “arrive” or the intransitive kir-e-ru “be cut” from its transitive counterpart kir-u “cut,” such errors are sensitive to the formation of morphologically marked vowel-ending (in)transitives by attaching the affix e to their unmarked (in)transitive counterparts (cf. Yanaike 2000).

Given these facts, errors in causative/inchoative alternation such as those in (24a,b) cannot be immediately taken as a piece of evidence in favor of the nonproductivity of potential verbs as argued by Fuji et al. (2008a,b). Instead the answer to the question (23) that we will try to pursue here is the unification of the formation of potential verbs and the morphological realization of (in)transitivity. More specifically, we would like to propose an account that the realization of the same morpheme e in the formation of potential verbs, on the one hand, and the derivation of vowel-ending verbs from consonant-ending verbs, on the other, is reduced to the availability of a specific functional head in the VP layer and the process of lexical insertion of the phonetic form e onto the head. On this view, consonant-ending verbs are not permitted as zero potentials simply because their VP structure lacks a functional head to overtly host the morpheme e. On the contrary, assuming that the morpheme e realized at the functional head in question also plays the role of forming (in)transitives of vowel-ending class of verbs in causative/inchoative alternation, it naturally follows that a verb which ends in e such as sim-e-ru “close” (which is the transitive counterpart of the intransitive sim-ar-u “close”) can be identified in itself as a zero potential form, while the attachment of the morpheme e is always required to derive an e potential verb from a consonant-ending verb.^[7]

Interestingly enough, Nakaishi’s (2016) observation is relevant here. Taking the examples of (25a,b), Nakaishi (2016: 82) states that “eru-transitives (i.e. transitives which end in the vowel e) cannot be morphologically distinguished from potential forms of consonant-ending transitive verbs and thus could be confusing in distinguishing between them, which is supposed to be among the reasons that there are cases eru-transitives are incorrectly used for potentials (i.e. e potential verbs).”

If the erroneous uses caused by the ambiguous status of the morpheme e in (25a,b) suggest that e potentials and eru-transitives share some grammatical properties for a child around the age of 3 years in that the morpheme e bears a certain set of features at a specific functional head onto which the PF insertion of the form e is applied, in our terms, two questions arise as to (i) what functional head in a VP layer could be responsible for deriving such a close correlation between the acquisition processes of potential verbs, on the one hand, and either transitive or intransitive verbs, on the other, all of which end in the same form e, and (ii) what property of the relevant head triggers the morphological realization of e.

Let us start with a brief review of our basic theoretical assumptions on the layered VP structure. We will adopt the framework of Distributed Morphology (Marantz 1997, 2001) to assume that the structure of VP in the traditional sense is layered as shown in (26) where (i) Voice introduces Agent of the verb and is also responsible for licensing accusative Case (Kratzer 1996, a.o.), (ii) v forms the complement of Voice and host (in)transitivizing suffixes (Pylkkänen 2002, 2008), (iii) Root is acategorial before it is merged with the first category-determining functional head but may introduce a Theme argument, and (iv) the top of the VP layer is optionally occupied by GET which licenses Experiencer or Beneficiary in its Spec (cf. Nakajima 2014, 2015).

Assuming with Nakajima (2011, 2014) that the functional category Get is promising in unifying the formation of potential verbs and vowel-ending verbs, we argue that the template schematized in (26) can deal with the two questions stated earlier.^[8] Nakajima (2011, 2014) proposed that (i) Get is a verbal functional category, diachronically grammaticalized from the lexical verb e-ru “get,” (ii) it occupies the topmost of the layered VP structure, and (iii) it licenses the subject of passives, causatives, and potentials in the Japanese language by assigning the role of Experiencer or Beneficiary to its Spec. What is crucial to our argument here is that the semantic nature of Get can be naturally characterized in terms of aspectual notions such as ACHIEVE, ARRIVE, or COMPLETE, which are inherited in part from the lexical property of the main verb e-ru “get.”^[9] In fact, this semantic consideration of Get is totally compatible with the fact that there are some dialects in Japanese (and other languages in the world as well) that the ability reading is closely related to the denotation of the aspectual end point of the event (Shibuya 2006). The following examples are from dialects in Northern Kyushu in western Japan. Northern Kyushu dialects have the potential form with kir-u “up,” an auxiliary verb grammaticalized from the lexical verb kir-u “cut,” which indicates the arrival at the end of the event; in our terms, lexical items such as kir-u “cut” used as auxiliaries for potentials are taken to be the morphological realization of the head of Get.

Turning our eyes to the standard Japanese, we can give an account of the difference in distribution between two different transitive verbs tsunag-u “connect” and tsunag-e-ru “link,” both of which have the common intransitive counterpart tsunag-ar-u “be connected.” Let us observe the following.

Contrasts seen in (28a,b) and (29a,b) show that the arrival at or the emergence of a certain resultant state (which is explicitly denoted in (29) by the occurrences of dative ni-phrase) is obtained in the clause with tsunag-e-ru whose comparatively strong implications for achievement or completion of the event can be seen in the types of the object it is compatible with. Again, the semantics of Get in connection with notions such as achievement, arrival, or completion are relevant here: the presence of the morpheme e in tsunag-e-ru brings about the effect from some resultant state of the event denoted is attributed to the subject with the role of Experiencer or Beneficiary, as schematically shown in (30).^[10]

Assuming that the occurrence of e at the head of Get can be taken ambiguously either as a potential morpheme or as a transitivizing marker of vowel-ending verbs, we can capture what we have observed concerning the acquisition of potential verbs by a child around the age of 2 years, as summarized in (31a,b).

5.2 Later stage (after the age of 2 years and 6 months)

As discussed in Section 4, as children reach the later stage of the acquisition of potential verb in addition to e potential verbs that they have already learned, they productively utter potential forms dependent on (r)are and begin to vocalize forms that deviate from standard in adult grammar, such as re-added expressions or e-rare forms, though with limited frequency. Also, in this period, it is clear that erroneous uses in causative/inchoative alternation are generally cleared up.

Putting e-rare forms aside for a moment, let us first observe the following data concerning the diversity in the form of potential verbs in (11a–c) and (12), which are repeated here as (32a–c) and (33).

Since Fuji et al. (2008a,b) simply take these expressions as deviant or even as ungrammatical for unclear reasons, they could argue that re-added expressions like (33) bring about some kind of redundancy caused by the potential morpheme e further merged to e potential verbs. Sentences such as those in (32a–c), nevertheless, cannot be ruled out immediately under the analysis by Fuji et al. (2008a,b) which assumes that the relevant potential forms are derived from consonant-ending verbs occupying the head of V just followed by the potential morpheme (r)are at the head of v.

What is to be noted here is the fact that (32a–c) are perfectly acceptable in adult grammar as potential forms in some dialects, including those of Kanto region, and that the re-added expressions as in (33) can also be used mainly in colloquial styles in Tokai region including Aichi and parts of Shizuoka, as mentioned earlier. In addition, Arai’s (2006) observation is of great interest that expressions in (32b,c) are uttered by a child whose parents are not from Kanto region and thus never employ the morpheme (r)are to form potential verbs from consonant-ending verbs, a situation comparable to the already mentioned case for the productivity of ra-dropped expressions in (22). It is admitted that materials accessible in analyzing the natural speech corpus of children are quite limited, and there remains difficulty in assuring the extent to which such examples have been observed uniformly. Nevertheless, there are good reasons to believe such dialectally limited expressions as those in (32a,b) and (33) to be derived through certain knowledge with regard to the structure of VP available in child grammar, in light of the fact that their utterances are not influenced by the so-called input factors in language acquisition including the living environment and the birthplaces of parents.

To account for the productivity of (r)are potentials and e-added expressions in child grammar, we will further propose that the form (r)are is decomposed into two independent formatives. Let us look at (34).

We have already argued that the morpheme e is the PF exponent of Get in the case of e potentials and vowel-ending transitivity-altered verbs as well. As for the other morpheme ar, it has been widely held in the literature since Kageyama (1996) that it is syntactically as well as semantically involved in the formation of inchoatives in Japanese (cf. Jacobsen 1992). Let us look at the following data, parts of which are taken from Kageyama (1996: 196):

The examples in (35) and (36) show that e and ar are realized as marking (in)transitivity of the verb. In accordance with (34), ar, which triggers the suppression or deletion of external argument, cannot co-occur with agentive by-phrases as shown in (35b) and (36b).

The structures for the (r)are potential ik-ar-e-ru and its re-added version ik-e-re-ru that we propose are roughly illustrated in (37a,b).

(37)	a.	[GetP DP(Experiencer/Beneficiary)i [VoiceP [vP [√P √ ik ] v] ar ] e ]	(ik-ar-e-ru)
	b.	[GetP DP(Experiencer/Beneficiary)i [VoiceP ec (Agent)i [vP [√P √ ik ] e ] Voice] e ]	(ik-e-re-ru)

In (37a), the morphological realization of ar at Voice makes external argument in its Spec implicit, and the subject is licensed as Experiencer/Beneficiary at the Spec of Get. On the contrary, in the absence of ar at the Spec of Voice, the morphosyntax of ik-e-re-ru in (37b) will never be subject to such an agent suppression: the subject can be construed as Agent as well as Experiencer/Beneficiary. This correlation between the presence of ar at Voice and the licensing of Agent at its Spec is supported by the fact that there indeed exist speakers of Yamagata or Akita dialects who prefer e to (r)are as the potential form in expressing the subject’s ability to execute an activity denoted by the verb (Shibuya 2006).

Note that in (37b) we posit the first occurrence of e at the head of v. As stated earlier, we follow Pylkkänen (2002, 2008) that (in)transitivizing suffixes are inserted at the head of v, which consists of the complement of Voice. Since the origin of forming of potential verbs from consonant-ending verbs with the aid of the suffix e in Middle Japanese was to invent a class of intransitive verbs denoting the property of the subject (e.g., nom-e-ru “drinkable” derived from nom-u “drink” followed by e), we have good reasons to assume that the morpheme e which is taken as the (in)transitivizing suffix as well as the potential marker can occur either at Get or at v in the proposed VP layer. This assumption is also in perfect accordance with the fact that a child around the age of 3 years or older does not completely distinguish between transitives and intransitives morphologically but still begin to utter (r)are or re-added forms for potentials.^[12]

Note also that the proposed structures in (37a,b) can correctly predict that a child at this stage of language acquisition wrongly produces the second type of erroneous uses, e-rare forms, which we argue has a structure as in (41). The relevant data in (13a,b), (14a–d), and (15) are repeated here as (38a,b), (39a–d), and (40).

(38)	a.	Ik-e-rare-nai	(3;8)
		go-Pot-Pot-Neg
		“I can’t go.”
	b.	Nug-e-rare-nai	(3;8)
		remove-Pot-Neg
		“I can’t remove my clothes”. (Okubo 1967: 146)
(39)	a.	Dasy-e-rare-nai	(3;0)
		pull-Pot-Pot-Neg
		“I can’t pull it out.”
	b.	Mat-e-rare-ru	(3;7)
		wait-Pot-Pot-Pres
		“I can wait.”
	c.	Hakob-e-rare-ta	(3;11)
		carry-Pot-Pot-Past
		“I was able to carry (it).”
	d.	Oyog-e-rare-ta	(4;3)
		swim-Pot-Pot-Past
		“I was able to swim.”
		(Shibuya 1994: 30)
(40)	Huk-e-rare-nai		(4;7)
	wipe-Pot-Pot-Neg
	“I can’t wipe (it).”
	(Arai 2006: 9)
(41)	[GetP DP(Experiencer/Beneficiary)i [VoiceP [vP [√P √ ik ] e ] ar ] e ]	(ik-e-ar-e-ru)

Concerning the reason that the potential morpheme e doubly occurs at v and Get simultaneously, we would like to suggest a possibility that a child at this stage of language acquisition begins to fully and randomly utilize the functional categories available to detect morphologically possible combinations among them. To support this view, let us compare (41) with the partial structure of an acceptable potential form ag-e-rar-e “to be able to raise” below, in which the first occurrence of e functions as a transitivizing marker and the second one a potential morpheme.^[13]

(42)	Taro-wa	omoi baaberu-o	ag-e-rare-ta.
	T-Top	heavy barbell-Acc	lift-vt-Pot-past
(43)	[GetP DP(Experiencer/Beneficiary)i [VoiceP [vP [√P DP(Theme) √ ag ] e ] ar ] e ]		(ag-e-ar-e-ru)

Under the analysis by Fuji et al. (2008a,b), on the other hand, the only possible derivation would be that the potential morpheme (r)are at the head v is incorrectly merged to the e potential verb at the head V. What is to be noted here is that all of the e-rare expressions in the relevant examples earlier and other 30 or more cases from the corpus data are from consonant-ending verbs, whereas we can find only two e-rare examples based on verbs which end in e. This contrast between the two types of verbal morphology concerning the possibility of producing e-rare forms is obviously problematic to the analysis by Fuji et al. (2008a,b), on a par with the contrast between zero potential forms and e potential verbs in the early stage of language acquisition we have seen earlier. If e potential verbs are treated as occupying the head V regardless of the conjugation type of their underlying verbs, the contrast in question should never arise, and even examples such as narab-e-re-rare-ru in Shibuya (1994) would be wrongly predicted to occur with much higher frequency. To sum up, Fuji et al. (2008a,b) again fail to provide a successful explanation either to the data from the initial stage of the acquisition of potential verbs as seen in Section 5.1 or to the data from the later stage discussed in this section.