Ruszkiewicz's "common errors"

When it comes to Ruszkiewicz’s “common errors”, there were a few that I had trouble with. I’m always confused about when to use “who” or “whom” in a sentence. I have never been able to tell the difference and still have a hard time deciphering which to use. Sometimes I struggle with knowing where to put apostrophes and when to use them. I feel like it can be difficult to know what pronouns to use with the correct antecedent. I don’t seem to struggle with comma splices, run-ons, or fragments when writing. Also, I don’t find subject/verb agreements to be too complicated. Those were the common errors I don’t have problems with.
-Shelby