“I” is singular, so why does it take plural verbs? Here's we say “I go to the store on Fridays” instead of “I goes to the store on Fridays.”
Why Do Words Change?
How did “demest” lose that “-st” suffix? Easy! Once speakers started using the plural “you” for both singular and plural, it was natural to use the same plural verb forms with it that they’d always been using.
The same thing happens today when speakers use “they” as a singular pronoun. They don’t say, “Does anybody know what they wants to order?” They say, “Does anybody know what they want to order?”
A webpage for a college class on the history of English talks about this development in second-person verb forms, and says that the poet Alexander Pope objected to using a plural verb form for “you” when it referred to an individual. According to the webpage, “for some time he used the pattern ‘you was,’ but abandoned it when his contemporaries condemned it as inelegant and vulgar.” These days, of course, grammarians don’t consider the “go” in “You go to the store” a plural form anymore. It’s second-person singular, or depending on your point of view, a default verb form that can perform the function of second-person singular.
And that’s how we ended up with just two verb forms for most present-tense English verbs—why we say things such as “I go to the store on Wednesdays.”
Thanks to Aaron for an interesting question. I’ll put the link for that class webpage on the transcript for this episode. It also gives an interesting sociolinguistic history of how and why “you” edged out “thou.”
þu demst; demest
he demþ; demeþ
David Crystal. The Cambridge Encyclopedia of the English Language. 1995, Cambridge University Press, p. 44.