The preservation of some collections of late fifteenth-century private correspondence – like the Paston letters, the Cely letters, or the Stonor letters– offers a very useful corpus to carry out quantitative sociolinguistic analysis, as they involve writers of different sex, age, social extraction, and geographical location. The historical and philological interest of these documents is outstanding, not only because they offer data on the political and domestic history of fifteenth-century England, but also because they were composed at a crucial period in the development of the English language (during the expansion of the Chancery English variety). In the Paston Letters , William Paston II represents the social manifestation of the development of the awareness of a well-established standard with his ‘Memorandum on French Grammar’ (Letter 82), written between 1450 and 1454. This is an exceptional document that provides us with a description of the English language of the late Middle English period by a non-standard user, which highlights the covert versus overt prestige motivations in his contradictory sociolinguistic behaviour and in the social psychology of that late Middle English speech community and society. The aim of this paper is to illustrate this contradictory sociolinguistic practice and the awareness of prestige patterns in the late Middle English period with quantitative and qualitative analyses of his use of past be forms, as part of a larger project on medieval and contemporary was/were -levelling in East Anglian English.