I think one-tense-prior rule is not that strict, so "had learned" here can be replaced by "learned" only, right?
gz45)I didn't laugh at the joke because I had learned it at the last meeting.
... had heard, not had learned.
You could use simple past, but the past perfect sounds more natural to me.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.