Can we say "you is" instead of "you are"?
"You is smart"
Is is singular, whereas are is plural.
Here is the simple answer from a question asked on ELU:
"you" is always plural. Or at least, it was so historically, and that has carried over into the present in verb conjugation.
The other answer that post offers:
is that "are" is the form of "to be" used for first person plural, third person plural, and both plural and singular in second person (with you). Thus, "are" with a singular "you" is also singular. It just looks exactly like the plural form. The same goes for "were" in the past tense, or for any other verb in second person: The form of the singular is identical to that of the plural.
Although I do think this isn't as simple.
In modern times, you is has crept in as a colloquialism (presumably stemming from this confusion), but is considered incorrect.