Permission, Prohibition, Obligation, No obligation
To express permission, prohibition, obligation and no obligation we usually use modal verbs.
Can is often used to ask for and give permission.
- Can I sit here?
- You can use my car if you like.
- Can I make a suggestion?
We can also use may and could to ask for and give permission but can is used more
Both can’t and mustn’t are used to show that something is prohibited – it is not allowed.
- You can’t park here, sir.
- You can wear jeans but you can’t wear trainers in that bar.
- You mustn’t speak when the teacher is speaking.
Can’t tells us that something is against the rules. Mustn’t is usually used when the obligation comes from the person who is speaking.
Have to and must are both used to express obligation. There is a slight difference between the way they are used.
Have to shows us that the obligation comes from somebody else. It’s a law or a rule and the speaker can’t change it.
- Do you have to wear a uniform at your school?
- John can’t come because he has to work tomorrow.
- In Britain you have to buy a TV licence every year.
Must shows us that the obligation comes from the speaker. It isn’t a law or a rule.
- I must call my dad tonight.
- You must hand in your homework on Tuesday or your mark will be zero.
- You must come and visit us the next time you come to London.
We use don’t have to to show that there is no obligation. You can do something if you want to but it’s not compulsory.
- You don’t have to wear a tie in our office. You can wear a tie if you want to but it’s OK if you don’t.
- It’ll be nice if you do but you don’t have to come with me if you don’t want to.
- You don’t have to dress up for the party. Wear whatever you feel comfortable in.
Modals – deduction (present)
We use modal verbs to say how sure we are about something.
We use must when we feel sure that something is true because there’s very strong evidence.
- He must live near here because he comes to work on foot. We don’t know where he lives but we’re sure it’s not far away.
- Come inside and get warm – you must be freezing out there.
- You’re a zookeeper? That must be very interesting.
Notice that must is followed by an infinitive without ‘to’.
2 might, may, could
We use might, may or could to say that we think something is possible but we’re not sure.
- Did you hear that? I think there might be a burglar downstairs. She’s not sure there’s a burglar but she thinks it’s possible.
- We’ll try to get there early but we may arrive late if there’s a lot of traffic.
- Don’t put it up there. It could fall off and hit someone.
Might, may and could are also followed by an infinitive without ‘to’.
We use can’t when we feel sure something is not true.
- It can’t be a burglar. All the doors and windows are locked. He doesn’t know it’s not a burglar but he feels sure it’s not.
- It can’t be far away now. We’ve been driving for hours. Where’s the map?
- Really? He has to work on Christmas Day? He can’t feel very happy about that.
Like the other verbs, can’t is followed by an infinitive without ‘to’.
Remember that all of these modal verbs – must, might, may, could and can’t have other uses. These are covered in another section.
Modals – deduction (past)
In the same way that we use modal verbs to say how certain we are about things in the present we can also use them to speculate about the past.
Have + past participle (‘have done’, ‘have been’ have stolen’ etc.) is called the perfect infinitive. When we use modal verbs to talk about the present they are followed by an infinitive without ‘to’. When we use modal verbs to talk about the past they are followed by a perfect infinitive.
must + perfect infinitive
We use must + perfect infinitive when we feel sure about something in the past.
- You must have been delighted when you heard you’d won the lottery.
- The thieves must have come in through the window. Look – it’s still open.
- Oh no! Where’s my car? Someone must have stolen it!
might/may/could + perfect infinitive
We use might, may or could with the perfect infinitive to say that we think something was possible but we aren’t sure.
- The thieves might have escaped by car but we can’t be sure.
- He should be here by now. He may have been delayed by a traffic jam or something.
- I can’t find my purse. I could have left it in the supermarket but I just don’t know.
can’t + perfect infinitive
We use can’t + perfect infinitive when we feel sure something didn’t happen in the past.
- I thought I saw John in town this morning but it can’t have been him – he’s in Greece this week.
- I can’t have left it in the supermarket – I had it on the bus on the way home.
- You can’t have read the instructions properly. They’re perfectly clear.