In Tae Kim's guide, he wrote a few example sentences:
In these examples, I don't really understand why some sentences use が and some use は.
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It only takes a minute to sign up.Sign up to join this community
All of these sentences on their own would permit either が or は. It is context that could force one or the other. This question is worth reading if you're interested how context comes into play.