I've learnt eight parts of speech since I was in middle school. Among them, conjunction and preposition have loads of things to memorize, just in my opinion. In this course, one of the important sentences for me is ''Thomas will be late to work, for he has an appointment.'' from coordinating conjunction lesson.
I've memorized using ''for+noun or noun phrase''. I've forgotten that for can be used between two sentences to indicate the reason.