

CAN & CAN'T
Ability
Possibilty
The florist can deliver the bouquet early.
Permission
Can I take day off?
Requests
Can you tell me the way to the church?

COULD & COULDN'T
Past ability
I could run ten miles in my twenties.
Possibility
He could be one who stole the money.
Polite request
Could I borrow your pen?

SHOULD & SHOULDN'T
Giving advice
You should take some medicine when you feel sick.
You shouldn't eat candy when you have a toothache.

MUST & MUSTN'T
Necessity
You must fill in this form.
Strong recommendation
You must take some time off.
Certainty
That must be Tom.
Prohibiton
You mustn't smoke here.