“You were” is grammatically correct in English. “You was” is nonstandard English which means that it is used in some parts of the English world.
“You were” is the second person singular of the verb “to be” in the past simple tense.
You were at the party last night.
Were you with John?
“You was” is incorrect and should not be used in English. You will hear people saying “You was” in different parts of the English-speaking world(Mostly in the North of England in my experience).
Do not use “you was” if you are a non-native English speaker as people will try to correct your English. Many people in the North of England use “you was” instead of “you were”.
There are also people in the United States who use “you was”.
How do you use “you were” in a sentence?
You can use “you were” in a sentence when you need a past simple form of the verb “to be”.
”You were” is usually at the beginning of a sentence or clause.
If you want to ask a question with “you were” then all you need to do is to invert the subject and the verb(Were you….?).
I told you that you were beautiful.
Were you at the cinema yesterday?