Is a verb a singular?

Use is with singular subjects and are with plural subjects. Collective nouns usually take is, but you can use are if you need to emphasize the individuals who belong to the group. Phrases like a number of… usually take a plural verb.

Why you is singular and plural?

You is always definite even when it is not specific. Semantically, you is both singular and plural, though syntactically it is always plural: it always takes a verb form that originally marked the word as plural, (i.e. you are, in common with we are and they are).