My brain is hemorrhaging trying to figure out how to properly use は and が. In school, my teacher told me that は is used when you're talking about a non-exclusive subject and が is used when you're specifically talking about one thing or subject. She also mentioned that は is used to introduce a new subject. It sounded easy enough to me but I just did a particle exerise using は and が and I pretty much failed it. Could someone help explain how to use these particles properly?