The following is a list of things that may go wrong in a writing assignment, particularly a mathematical writing assignment. Please have the following comments in your mind when you are typing your homework.
Theorem:...
Proof:...
Corollary:...
Proof:...
You'll probably want some text before and after, and perhaps some in between, just to ease the transitions. Also along these lines, it's nice to have some sort of “end of proof” signal, such as a “QED” or a black box.