You DON'T HAVE TO attend the debate if you don't want to.
You MUSTN'T go past the fence over there.
Lee MAY arrive later, but he's in a traffic jam at the moment.
She SHOULDN'T take that job because she'll be bored.
We AREN'T ABLE to get into the lecture hall yet.
You CAN find the details at this website.
COULD you help me with my CV later, please?
He COULD ski really well before the accident.
You HAVE TO find more up-to-date information for the report.
They WEREN'T ABLE TO finish the application in time.
possibility
obligation
inability (present, future)
ability (past)
advice / suggestion
lack of obligation / necessity
inability (past)
ability (present, future)
prohibition
permission / request