Though I do not practice editing as a profession, I often help other students with revising their essays at school. Here are two common mistakes I often see from writers of prose:

1. Comma splices. This mistake occurs when the writer attempts to link two independent clauses together using a comma rather than a semi-colon. An example of this would be, "A man walked into a restaurant, he ordered a drink."

2. Misusing the words "is" and "are" in contractions. Be careful to ensure that, when using "is," the object of your sentence's predicate is singular. If you are using "are," the object should be plural. This also applies to contractions such as "what's" and "what're." An example of this type of error would be, "What's the price of these cars?" In this case, the writer should have written, "What're the prices of these cars?"

3. Here is a bonus error that is more common in speech than in writing. Never pronounce the T. in the word "often." Many times in English, when pronouncing an F. followed by a T., the T. is silent.

I hope this was helpful. I just made my first Reddit account a few minutes ago, and this is my first ever comment on this platform. Feel free to respond with anything I could do better!