1. Do what you can to make it sound like normal English. Even if it doesn't make any sense next to the sentences on either side, you at least need to make that individual sentence sound correct. If it's so bad that you're afraid you'll further confuse the meaning, proceed to step 2.
2. Regardless of whether you've fixed the sentence or not, if it still seems unclear, leave a comment on it. Try to ask a question that at least leads the author in the right direction for clarifying the sentence. If all else fails, leave them a polite comment like, "I'm not sure what you mean in this sentence."
Not at least leaving a comment makes it look like you ignored the sentence!
