“They were” is grammatically correct in English. “They was” is nonstandard English which means that it is used in some parts of the English world.
“They were” is the third person plural of the verb “to be” in the past simple tense.
They were at the party last night.
Were they with John?
“They was” is incorrect and should not be used in English. You will hear people saying “They was” in different parts of the English-speaking world(Mostly in England in my experience).
Do not use “They was” if you are a non-native English speaker as people will try to correct your English.
How do you use “They were” in a sentence?
They can use “They were” in a sentence when you need a past simple form of the verb “to be”.
”They were” is usually at the beginning of a sentence or clause.
If you want to ask a question with “They were” then all you need to do is to invert the subject and the verb(Were they….?).
According to Merriam Webster, you should also use “they were” for people who prefer to use the pronoun “they” instead of “he or she”.