Which one is correct: "If I were a cop, I would wear a uniform" OR "If I was a cop, I would wear a uniform"?
if I were
This we call the second conditional utilising the subjunctive form, which can be used to discuss hypothetical/unreal situations.
If I were a cop, I would wear a uniform implies that I am not a cop and so I don't wear a uniform.
if it was
This would be the beginning of a question about a past event, for example:
If it was raining, why did you go out? meaning that it rained and you went outside; the speaker is questioning your motives.
Hope that helps
But as to correctness, both are "correct".
There is also:
I think "were I" sounds very elegant, especially the passive "Were I to be". I'd like to find opportunity to use this in speech and get away with it without it sounding forced.