Profs go nuts when students seem to make the same mistakes over and over. What a lot of these teachers don't realize is that students can't decode those scribbled comments in the margins.
Sometimes, profs give students the chance to revise a paper for a better grade. When they do that, it's a good idea for the student (like yourself, perhaps), to read through the teacher comments as soon as the teacher hands back the papers. I know it's a tense time, with some people freaking out about their grades while other people just shove their papers in their folders without seeming to even look at them. Heck, when I was an undergraduate student, I thought it would be considered tacky or bad manners to check my grade in front of people--sort of like counting money in front of an old friend. Nonetheless, it's very, very important to let your professor know that you're reading (or trying to read) his or her comments. Trust me. Teachers want to feel useful.
So... when you get a paper back, read through the comments right away. If there's anything you don't understand, get in line or crowd around the professor's desk at the end of class. When the instructor tells you what those weird comments mean, write that stuff down in the margins. It will save you time if you decide to revise that paper for a better grade, it will make your professor happy with you, because it will show that you actually want to improve, and it just might improve your writing.
If you want to check out some common proofreading marks that professors like to use, try this link, courtesy of Capital Community College in Hartford, Conn.: