The kinds of pronoun, choosing the right case (subject, object, possessive), making a pronoun agree with its antecedent in number and person, avoiding vague or ambiguous reference, and the who-versus-whom and that-versus-which decisions.
Reading the 9 sections above, the worked examples and the practice set is free — the whole knowledge base is on the free plan, along with flashcards, private notes and Ask Onely.