When your reader says something like: Some of your sentences seemed confusing
First make sure your reader has enough in common with the ideal audience for your paper. If, for example, you’re writing a paper for a course that your reader has not taken, you might be using the vocabulary and concepts of that course in your paper, and your reader is only confused because they don’t know enough about the subject.
Find another reader, closer to your ideal reader
If your reader is a member of the “audience” for your paper, then you need to make sure that your sentences are clear. Read more about revising unclear sentences.