I am no pro at this but...

I looked at a bunch of homework submissions that had received official feedback. It's all about the shadows created by the forms. I filled in the center portion as best I could. Then moving towards the right I thought about how with more directly light there would be less shadow, then barely any shadow or marks at all when the light is most strong and direct. And then oppositely, to the left as there is more oblique light there will be more darkness, more shadow, and smaller pockets of lit area. I doubt any language issues are the problem. Your English seems absolutely fine. I think it's a thinking and visualizing and building-a-form-only-with-shadows problem, which is to be expected. I know it was hard for me. But it was also fun and rewarding to stretch my brain.