At least they are trying

If they get too good we'll lose all of our fun content

I have just finished a college course operational English. used to do well, But I want to actually look at parts of speech in a sentence and understand enough to know for sure why I am using who or whom or when to choose subjective or objective pronouns. (instance: do you consider it was THEY who left the door unlocked overnight?) My textbook spent more time suggesting to substitute he or him for who or whom, But I wanted to be able to know very well what part of speech was responsible for the choice. What is the ideal college level or above textbook to teach me this? Thank you upfront.

Perhaps that isn't a big worry :)