Many English speakers are confused about the usage of the words ‘can’ and ‘may’. For example, ‘Can I drink water?’ is incorrect. ‘May I drink water?’ is the correct phrase to use in this case.
The key difference between ‘can’ and ‘may’ is that ‘can’ talks about ability and ‘may’ talks about permission.
Can is used in two cases:

To talk about ability.

  • - I can finish my homework by 5 pm.
  • - Can you finish your homework tonight?

To ask or give permission informally.

  • - Can I use your pen? (To a friend)
  • - You can use my pen. (To a friend)

May is generally used to ask or give permission formally.

Let us take a situation between a student and a teacher.

  • - May I drink water?
  • - Teacher: Yes, you may.

Let us take a situation between two strangers.

  • - May I borrow your pen?
  • - Yes, you may.
Difference in the usage of 'can' and 'may' in the English language
Also see