We use must or have to (which is not a modal verb) to talk about obligation.
- You must arrive on time.
- You have to look good on a date.
We use needn’t and don’t have to (which are not modal verbs) when there is no obligation.
- You needn’t pay for everything on a date.
The verb need (without to) is only used in the negative and interrogative. In the affirmative, we use need to to express necessity.
- You need to buy my boyfriend a present today.
We use mustn’t to talk about prohibition
- You mustn’t cheat on your boy or girlfriend.
Modals of ability and permission
We use the modal verb can to talk about ability in the present.
- My girlfriend can ride a motorbike.
We use the modal verb could to talk about ability in the past.
- I couldn’t drive last week, but I can now!
As can and could cannot be used to talk about ability in all tenses, we use be able to (which is not a modal verb) in other tenses.
- Soon Peter will be able to speak Spanish fluently.
We use the modal verb can to talk about permission in the present.
- Can we go?
As can cannot be used in all tenses, we use be allowed to (which is not a modal verb) to talk about permission in other tenses.
- You were allowed to stay out late last Saturday.
We can also use could to talk about general permission in the past. However, we cannot use could to talk about permission for a particular action in the past. Instead, we use the correct form of be allowed to.
- Yesterday evening, I was allowed to watch a cartoon.
Modals of possibility and certainty
We use may, may not, might, might not and could to talk about present or future possibility.
- They could be on the train now.
We use can’t when we believe or guess that something is impossible.
- He can’t be boring.
We use must when we are certain something is true.
- He must be a nice person if he’s Jeff’s friend.
We use must and can’t when we make logical deductions.
- That must be his brother.
Modals + perfect infinitives
Can’t have + past participle
- We use can’t have + past participle to express an impossibility in the past
Could have + past participle
- We use could have + past participle to suggest an alternative past action, even though it is now too late.
Could/may/might have + past participle
- We use might, may or could have + past participle to talk about something that was possible in the past.
Must have + past participle
- We use must have + past participle to express a certainty or to make a logical deduction about the past.
Should have + past participle
- We use should have + past participle to give an opinion about past events, even though it is now too late.
Shouldn’t have + past participle
- We use shouldn’t have + pat participle to express regret or criticism about past events.
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