We find the subjunctive form confuses a lot of writers. Let's help to clear up the confusion.
Guideline: Use were (instead of was) in statements that are contrary to fact.
Closer look: Statements contrary to fact, especially those that begin with "if," call for a special form of the verb known as the SUBJUNCTIVE. Careful writers and speakers use subjunctive forms in just the right situations, particularly in if clauses that express a statement contrary to fact. The main change you need to make in most of these situations is to substitute were for was.
If this were [not was] a real Florida tourist attraction, that waterskier would have made it over the jump! (The subjunctive form of to be [were] is the proper choice because the statement is contrary to fact: This is not a real Florida tourist attraction.)
If I were you, I wouldn't tempt those alligators by swimming here. (Were is the proper choice because the statement is contrary to fact.)
I have often wished that I were more like Dick Pope, the father of waterskiing. (Were is the correct choice even though the main verb is in the past tense. The statement is still contrary to fact.)
I wish I were the star waterskier at Cypress Gardens. (Were is the proper choice because the statement is contrary to fact.)
If it was raining yesterday in Winter Haven, the waterski show was probably called off. (Was is the proper choice here because there is a chance that it was raining.)
The only reason I called was to see if the number-one-skier job was still available. (The verb here is not in the subjunctive mood because the idea following "if" is not contrary to fact. The job being open is a distinct possibility.)
Source: Grammar For Smart People