Why do we say "you are" instead of "you is"?

[Thudlow_Boink]

I just discovered that Merriam-Webster has a good explanation of [url=How did you become singular?

As far back as the 14th century, the plural forms ye and you began to be used to address one person—usually a superior—as a mark of deference and respect. This change could have been influenced by the first-person plural we (the royal “we”) used by sovereigns or reflected the impact of French politeness at work in Middle English.

Initially, the distinction between singular you and plural you was signaled by verb agreement; you was for the singular continued in polite if informal use well into the 18th century before it lost respectability.

[PatrickLondon]

@Andy_L

It’s complicated. English used to have a singular 2nd person (thou) and a plural 2nd person (you/ye). Thou became the singular used for intimates and inferiors; people used “you” on formal occasions to single persons (the “royal we” implied a royal “you”). The intimate form died out, leaving “you” to work as singular and plural - but it kept the original association with plural verb forms.

And the old intimate form was “thou art” - not so big a change then to switch to “you are”.