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According to Google Search, "if it wasn't for you <verb>ing" and "if it weren't for you <verb>ing" are both used.

What is the correct usage of wasn't or weren't in this case?

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In English the Past subjunctive mostly has the same form as the normal Past (indicative). That's why Pt* (Past subjunctive) is only used with special indicators, after words as, for example, "if, as if/as though" + would in the main clause. The only verb that has special forms for Pt* is to be:

Past was/were/was - were/were/were, Pt* were/were/were - were/were/were

But as Pt* has mostly the same form as normal Past, in colloquial speech the genuine subjunctive "you were*" is mostly replaced by "you was". This is a bit earth-bound and in more elevated style you still use the proper subjunctive "you were*".

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