- CAN : ability and permission in the present.
- COULD: ability and permission in the past.
- SHOULD: to give advice or opinion.
- MUST: to talk about a strong obligation.
- MUSTN'T: to say that something is prohibited.
- HAVE/ HAS TO: to talk about obligations.
- DON'T/DOESN'T HAVE TO: to talk about things that aren't necessary or obligatory.
