Writing in English is not as simple as just using English words. Especially in academic writing, you need to learn to organize your thoughts and ideas in ways that English speakers (and your professor in particular) are expecting them to be organized.
If this seems like a strange idea, you really need to watch this video. In all seriousness, this may be the most important information you ever get about how to write an English essay.
I wouldn’t say that if I didn’t think it wasn’t true!