Surprisingly often they don't know. If it's not, that's because some fuzz has worked its way into the machinery. I don't mean that some people are born clearheaded and are therefore natural writers, whereas others are naturally fuzzy and will never write well. Good writing doesn't come naturally, though most people obviously think it does. I doubt it.
Writing is hard work.
A clear sentence is no accident. Very few sentences come out right the first time, or even the third time. Remember this as a consolation in moments of despair. If you find that writing is hard, it's because it is hard. It's one of the hardest things that people do.