Strictly speaking, you is a plural pronoun which takes a plural verb. Since the singular thou is no longer used in prose and conversations except poetry, the only option left is to use you for singular pronoun and plural pronoun.But it has not lost its plurality as far as the verb that follows it is concerned. so we say:
you are a new user
You write well.
You are friends
You are a friend of mine
If you is treated singular it shoud be you walks but it is not.
So inorder to avoid the ambiguity we say;
you are all welcome sinnce you are welcome may refer to one person or many persons.
You will have to know that singular they and plural they exist in English.
They are friends( plural they)
They are a new user( singular they)
We have to know that you and they take a plural verb as they are basically plural pronouns.The verb does not lose its property. plural pronoun takes a plural verb and singular pronoun takes a singular verb
So we can never say:
you is a teacher
They is a new comer.
But I is an exception to this rule.
I is followed by a verb without inflection and takes the basic form of the verb in the simple present and am in the present continuous and when it is used to talk about states.It takes a singular verb in the past tenses.
I speak English
I was playing cricket.
I was at home
I am playing cricket
I is a singular pronoun but it behaves peculliarly as far the verbs that follow in present and past tenses are concerned.