This study examines the modal adverbs in English including doubtless, indeed, no doubt, and of course from a functional perspective. Although an increasing number of studies have investigated “core” modal adverbs (e.g., certainly, possibly, probably), a comprehensive analysis including these four modal adverbs has not been offered in modal adverb studies. The present study begins to address this research gap by providing descriptions of these adverbs using data from the LOB (1961) and FLOB (1991) corpora. To adopt a more comprehensive view of the set of modal adverbs, I compare the behaviors of the modal adverbs without -ly to those with -ly in terms of their discourse and interpersonal functions in the text, and present the relationship between form and function of the modal adverbs. The results of the analysis demonstrate that the modal adverbs that convey the same degree of probability fulfill different functions at the discourse-pragmatic level. Moreover, the modal adverbs without -ly have increased their pragmatic functions over time, thus signaling a short-term diachronic development. Finally, I provide an explanation of this change in terms of grammaticalization and pragmaticalization.