'can' and 'may' : the difference

There is often confusion over the words 'can' and 'may'.

+ Can:
The word 'can' is used to denote ability.

"I can swim."
(I have the ability to swim.)

"Can he lift 150 kgs?"
(Does he have the ability to lift 150 kgs?)


+ May:
The word 'may' is used to denote permission.

"You may swim in this river."
(You are allowed to swim in this river.)

"May I have a biscuit?"
(Am I permitted to have a biscuit?)


+ Can for Permission (Informal Setting):
These days, however, the word can' is used for both ability and permission, particularly in an informal setting.

"You can swim in this river."
(You are allowed to swim in this river.)

"Can I have a biscuit?"
(Am I permitted to have a biscuit?)


+ Always May for Permission in a Formal Setting:
In a formal setting or in polite company, you should use 'may' to denote permission.

"May I leave the table?"
(Am I permitted to leave the table?)


NOTE - AUXILIARY VERBS:
'Can' is an auxiliary verb meaning 'to be able to'. 'May' is an auxiliary verb meaning 'to be permitted to'.

"Can I go outside, grandma?"
"You can. You're just not allowed."


TAKE THE TEST:
Select the correct version:

"Can / may I have a lollipop?" (Alternative: Am I allowed a lollipop.)

"I say, can / may you catch the bus to town from the riverside pub?" (Alternative: Does the bus go to town from the riverside pub?)

"She can / may sing like an angel." (Alternative: She has the voice of an angel.)

http://www.grammar-monster.com/easily_confused/can_may.htm