There are a few unknowns regarding this particular question. The prompt is correct that a tremendous amount of communication happens nonverbally. This makes written-only communication much more difficult. What isn't clear about this question is whether or not the online classroom environment is a written-only environment. Many online classes include a video component through something like Zoom. This does allow nonverbal communication to enter into the learning environment, because the teacher can see the students and the students can see the teacher. Nonverbal cues like body posture, fidgeting, yawning, eye rolling, and more can all still happen with a video-chat classroom. If you are looking for a way to effectively communicate with the professor, this type of video classroom is an excellent option, because he/she can see your nonverbal cues.
I do completely understand that the video of these kinds of classes can be very poor, but that doesn't mean nonverbal cues are gone when everybody turns off their cameras. Nonverbal communication also includes how you say something. Your tone, vocal inflections, volume levels, and more are all parts of nonverbal communication, and those things still come through with just audio. Be present in those classroom experiences, because simply being there tells your professor about you level of commitment.
If written communication through emails and forums is the only type of communication possible, then effective communication comes down to word choice and punctuation. Don't end every sentence with an exclamation point! It might make you look excited! It also might make your reader think you are angry and yelling at them! Word choice is also important. Leave slang completely out of the written communication. Explain your position and point in a concise manner. Your reader doesn't want to read three paragraphs when your point could have been made in three sentences.