When it comes to verbs, may is quite the troublemaker. So how do you know if you should use may or if you should use can?
- You use the verb can when you are asking if something is possible.
- You use the verb may to ask if something is permissible.
Your teacher doesn't know if you can go to the bathroom. That's a pretty personal matter, so why would you even think about asking that? What you want to know is if you have permission to visit the little girls' or boys' room.
And yes, it's one of the most annoying things on the planet when you ask someone "Can I [fill in the action]?" and they respond "I don't know… can you?"
We love good grammar, but… don't be that guy.
Example
If you can, I'd love it if you picked up one of those stuffed-crust pizzas with extra anchovies on your way home from school.
The desirability of anchovies as a pizza topping is debatable. But one thing there's no debate about is the desirability of can in this sentence. We can assume the speaker has permission to pick up the pizza, and that's not really the issue here anyway.
What's in question is whether he's able to pick it up. Does he get off work in time? Does he have enough money and gas to get it? Has he been cut off from the pizza place by a water main leak? So many questions surround his ability to pick up that pizza that can is practically crying out "Pick me, pick me!"