Modal verbs
A basic explanation of Modal Verbs can be found in the Pre-Intermediate Section. Here are some more advanced and subtle differences to note:
1) CAN / COULD / ABLE TO
COULD is sometimes the PAST of CAN
I'm sorry that I couldn't come yesterday – I was delayed.
When he was twenty, he could run a mile in under four minutes.
COULD / WAS ABLE TO
COULD is used to express general ability:
Dawson was the best boxer I ever saw – he could beat anybody.
WAS ABLE TO is used to talk about what happened in a specific situation:
We were lucky that we were able to save so much money on that deal.
(Not "we could save so much money")
COULD also has a present and future meaning when making suggestions:
So, we could go to dinner now, or we could go later.
I am so sad, I could cry!
CAN is also possible here, but is more certain than COULD:
We can go now or we can go later.
COULD is also used like MIGHT:
Who's at the door? >>> It could be the postman.
Comets and asteroids could strike the Earth at any time.
COULD HAVE DONE is used for things that were possible in the past but didn't happen:
He was stupid to jump into the crocodile exhibit – he could have been killed!
You know you could have asked me for help when you were in trouble last week.
2) CAN'T / MUST
CAN'T is used to say you feel certain something isn't possible:
You can't be angry – I was only joking!
We just ate three cheeseburgers – you can't be hungry again.
MUST is used to say that you feel certain something is true:
You must hate your boss – he is so horrible to everyone.
I'm sorry your girlfriend ran off with the pizza delivery man – you must be devastated!
The PAST is CAN'T HAVE DONE / MUST HAVE DONE
She called you an idiot? She can't have been serious!
Jenkins? No, he's too slow, he can't have won the race.
She called you an idiot? She must have been joking!
I can't find my briefcase anywhere. I must have left it at the office.
3) MAY / MIGHT
The basic meaning of these words is, of course, "perhaps"
That may be true, but I'm not sure. (perhaps it is true)
That might be true, but I'm not sure. (perhaps it is true)
The PAST is MAY HAVE (DONE) and MIGHT HAVE (DONE)
Why didn't he come yesterday? >>> He may have been sick.
Why was he late? >>> He may have been stuck in traffic.
In the positivebform, COULD is very similar to MAY / MIGHT:
Who's at the door? >>> It could be Tom.
>>> It may be Tom.
>>> It might be Tom.
However, in the negative, COULDN'T is different from MAY NOT / MIGHT NOT:
The wedding was a disaster. She couldn't have been happy.
(It was not possible for her to be happy)
The wedding wasn't as good as planned. She may not have been happy.
(Perhaps she was happy, perhaps not – both are possible)
4) MUST and HAVE TO
MUST is used for personal feelings or decisions
That's the best film I have ever seen! You must see it as soon as possible!
I feel so fat – I must lose some weight!
HAVE TO is impersonal. Have to is used for facts, rules and situations
We have to turn around – this road is closed.
I have to wear these special protective gloves at work – we handle dangerous chemicals.
Sorry, but we can't join you – we have to visit my aunt in the hospital.
The good news is that you can also use HAVE TO for personal feelings or decisions as well:
That's a great film – you have to see it!
Mmm... this is delicious – you have to try some!
The best rule is, if you are not sure, use HAVE TO and you will always be right.
Also, MUST has no PAST form. In cases where you need to express the PAST, use HAD TO:
We must leave now.
We had to leave at noon yesterday.
And, MUSTN'T is totally different from DON'T HAVE TO:
Please, you mustn't tell anyone! I would be so embarrassed if you did.
(It is very important that you do not tell anyone)
You don't have to tell anyone about your mistake.
(It isn't required that you tell anyone – the choice is yours)
The past form of MUSTN'T makes use of the phrase BE ALLOWED TO, which can be used to express the present as well.
Present: I mustn't sign the contract. / I am not allowed to sign the contract.
Past: I wasn't allowed to sign the contract.
DON'T NEED TO is sometimes used instead of DON'T HAVE TO
We don't need to hurry, the train leaves in an hour.
It can be used in the past too:
I didn't need to go to the meeting. Last week, my boss told me I could miss it.
(It wasn't necessary, and I knew it at the time)
5) SHOULD
SHOULD is typically used to give advice or offer opinions:
If you are hungry, you should eat something.
My sister has had a bad day, so everyone should try to make her feel better.
You should stop smoking – it is so bad for your health.
d) The PAST of SHOULD is SHOULD HAVE DONE:
We should have done more to help him – now it is too late.
(We didn't do more, but it would have been good to do so)
I'm sorry you were hurt, but I told you that you shouldn't have tried to ride that moose.