I got a copy of Schoenberg's Fundamentals of Musical Composition and it's quite a good book. I recommend it to anyone seriously interested in composition (though it does come off as rather Vulcan as he emphasizes musical logic). One thing Schoenberg mentions a great deal is the notion of sentences and periods. He says that there are two basic kinds of themes, periods and sentences.

The period is built on an antacedent and a consequent, two phrases that usually parallel each other. The sentence is built on a phrase, a sequence of the phrase, and some form of development consisting of something he calls liquidation (or rather something like it). It was interesting because I had never heard of sentences before, though I had heard of periods and phrases. Any thoughts?

I disagree with Schoenberg, I dont think you can narrow it down to two kinds of themes... but then again I haven´t heard his arguments.