A friend ask me why there are movies or songs with the following phrase:
I said that it was slang. However, I've seen that people said this when they talk in singular form. Is it correct?
It is a colloquial form, used by those who are poorly educated or using regional and dialectical English. It is generally not acceptable.
The correct form is "you were" since technically "you" is a plural form which is now used for the singular as well as the plural. The singular form is "thee" which is only now found in very, very rare dialectical usage.