I have heard native English speakers using from time to time expressions like "I thought YOU WAS", or "I believed THEY WAS". I would like to know if this association is basically correct or just accepted due to the mass usage. If it is correct, is there a specific context when you can/should use this combination? Any formal rules?
Re: you was
It's non-standard English. The standard usage for Subject-Verb Agreement for the verb BE (am, is, was, were, etc) is as follows:
I believe you were
I thought they were
I believe I was
I thought I was
Note, what you've come across here is called "Paradigmatic Regularization". That's when speakers regularize irregular-verb paradigms, like this:
all the were's become was's
you were => you was
they were => they was
It's not only more efficient, it's a tell-tale sign of language change.
Re: you was
The formal rule is: It is incorrect grammar. "was" is the third person singular form, and that's all there is to say about that.
However, "was" is used for other persons as well in several non-standard dialects of English, for example in many London dialects. Because it is non-standard, it should not be used in formal English. You, as a student of English, should never use it unless you really know what you're doing.
There's an old joke on this subject. A teacher knocks on the door of one of her pupils. The pupil answers the door.
Teacher: Hello. Are your parents in?
Pupil: They was in, but they is out now.
Teacher: "They was in"? "They is out"? Where's your grammar?
Pupil: Oh, she's in the living room, watching TV.
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