Let's say that at the end of my opponent's turn, I've drawn no cards. I {Cloudshift} a {Captain of the Watch} with a {Mentor of the Meek} in play. I know I might draw {Entreat the Angels}, so I want to keep as much mana open for that as possible - but I also want to draw three cards if I can.
How exactly does Mentor's ability work? Do I have to pay {1} to get his ability to draw a card on the stack at all? Or does it go on the stack regardless, and I choose to pay {1} as it resolves? If it's the latter, then I can miracle Entreat on the first draw and get two more angels out of it, letting the other two triggers go unpaid, than I would if I had to pay for all 3 triggers immediately.
To further complicate it, let's say {Cathars' Crusade} is in play. I can order my triggers such that I draw and cast Entreat before the first Crusade triggers resolve, giving my angels an extra 4 counters, right?
The mentor's ability triggers when an appropriate creature enters the battlefield. You choose to pay {1} when the ability resolves. And yes, ordering your triggers correctly would allow you to pump up your angels. The crusade's ability gives the counters when it's trigger resolves.
RESOLVED
Glad my instincts were right. I have a pretty good understanding of triggered abilities now, I think. Thanks.
Glad to help! Just keep in mind that triggers check the game state for conditions relevant to their effect (in the case if the crusade, a creature etb and then how many creatures you control) twice: Once when placed in the stack, and once upon resolution. If you can remember that, you can use the stack to your advantage like a pro.