In conditional sentences we use the simple past to express an unreal present or future. "I wish I HAD a million dollars." Where is the million dollars? In my dreams! In formal grammar "were" is always correct for an unreal present. "If I were the president, I would lower taxes." Am I the president? No! I'm dreaming of being the president. But today I hear people use "was" and it's OK because everybody understands.

"was" is the past tense of "is". "Was" is used for singular. eg: I was talking on the phone yesterday. "were" is the past tense of "are". "Were" is used for plural. eg: They were playing football in the field just now. In another case: "If I were you I would help you." We use "were" here because we are talking about something that's not possible to happen. More examples: If I were a cat. If I were a pen. Hope this help you understand.

For basic past-tense sentences, use 'was' for singular and 'were' for plural (the exception being 2nd person singular 'you' which uses 'were'). But you always use 'were' for conditional (if...) sentences. The second and third conditionals are past tense and both present an unlikely or impossible situation. "If I were... if she were..." This extends to unreal situations, as Professor B points out. So "I wish I were..." is the correct usage. While "was" is commonly used in speech - people using the simple conjugation - you would lose marks in a written essay for that.