Practice!
Seriously though,
1. FV formula - use when question talks about stuff like what will the investment grow to after 20 years, or in 20 years time, or something like that.
2. little PV formula - it's just the reverse of the compound interest formula - use when the question says something like what single amount of money would need to be invested now so it will grow to a certain amount.
3. big PV formula - ALWAYS used for LOANS, and N is the amount of the loan - also used when the question says something like what single amount of money would need to be invested now INSTEAD of investing a certain amount each month or year or whatever.
That's about it I think.