I used to teach high school, and that's a job where you see a range of writing skills, from pretty darned good to downright awful. One mistake often made but easily corrected is starting sentences with the same word over and over. Particularly in passages where things are happening to a protag, it's difficult to avoid repeating "He" or "She" or in the case of first-person narrative, "I" as the first word of a sentence. This isn't limited to student writing. I've seen some big-name authors, probably writing to a deadline, who didn't go back and reread often enough to catch this natural (but annoying to the reader) tendency. To check for repetitive beginnings, simply choose a page at random and list or underline the first word of each sentence. You'll easily see if you've overdone a word, and you can rearrange sentence structures to correct the problem. It's not so much that you don't know, it's that you don't notice!