There's a few times in every pilots life when they should use the phrase "unable" -
1) you're going too fast to safely exit the runway at the taxiway that Tower has directed you towards
2) you can't accept a LAHSO clearance due to landing performance
3) you've been given a hold in the middle of icing conditions
4) ATC gives you a speed restriction that you can't maintain
5) in short... any time that you're given a clearance or instruction that affects the safety of flight or exceeds your aircrafts limitations
6) and lastly, definitely if you haven't had coffee on your way to pre-flight!
