It is a subtle difference and most native English speakers don't understand it. Maybe even I don't totally understand it. But here is my best explanation. 1. "If I were" is what is called the subjunctive mood. You use it to talk about things that are definitely not true. For example, I (an American) could say: "If I were Chinese, I would be very angry about what the graduation speaker said." Because I am not Chinese, we use "If I were." 2. "If I was" would be appropriate when talking about something that might be true. For example, let's say you are not sure whether you arrived late. You could say "If I was late, it did not matter because everyone else was late too."