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Language reference

Expressing ability, possibility and 3VMFT BOE PCMJHBUJPOT must BOE have to

obligation Use must and mustn’t / must not:
"CJMJUZ can could be able to t to state rules and laws, often in a formal context:
Meat packaging must comply with the new regulation.
t Use can/can’t for abilities in the present:
Motorists must not exceed 120 kph on the motorway.
Cats can see in the dark.
You mustn’t ride your bike without a helmet.
I can’t drive.
t to express a personal feeling of obligation or a personal
t Use could/couldn’t for general abilities in the past: belief that something is important:
When I was younger, I could run very fast. I must phone my sister today. I mustn’t forget.
I couldn’t walk until I was nearly two years old.
You must see this fi lm – it’s great!
t For ability to do something in a specifi c past situation,
we use the negative couldn’t, but we don’t often use the Use have to:
affi rmative could. Instead of could, it is usually better to t to describe a duty or obligation, often coming from an
use be able to, manage to or succeed in -ing: external source:
We couldn’t open the door with the key. Eventually we She has to be at a meeting at 8:30 tomorrow morning.
managed to break a window and were able to get in. Compare these sentences:

t For future abilities, use will be able to: I have to fi nish this report by tomorrow. (= This is
My little sister will soon be able to read and write. something that someone else is insisting on.)
I must fi nish this report by tomorrow. (= I myself feel that
t Use be able to for other forms where there is no option
this is essential.)
with can or could:
I’d like to be able to see better. He has to go to the police station. (= The police have
given this order.)
1PTTJCJMJUZ can BOE could He must go to the police station. (= I believe it’s essential

t Use can/could to describe what it is possible to do. Use for him to go.)
can/can’t for the present and future, and could/couldn’t Use don’t have to:
for the past. We also use be able to, especially for the
past and future: t to describe a lack of obligation or necessity:
Passengers can get to London from here in 35 minutes. You don’t have to go to the party if you don’t want to.
Where we used to live, we couldn’t get there by train. Compare these sentences:
We can / We’ll be able to discuss this at tomorrow’s We don’t have to use this machine. (= We can use it if we
meeting. want to, but it isn’t essential.)
t Use could (but not can and not couldn’t) for uncertain We mustn’t use this machine. (= We’re not allowed to use
future possibilities: it – it’s prohibited.)
I think it could rain later. (not can rain) /FDFTTJUZ
But It may/might not rain tomorrow. (not couldn’t rain)
t For necessity, use need to or have to:
t Use could have (not can have) + past participle for To get to the airport in time, we’ll need to / have to catch
uncertain past possibilities: the 4:30 train.
I don’t know where she went. I suppose she could have
gone to the supermarket. t There are two negative forms of need:
We have plenty of time, so we needn’t hurry / don’t need
t Use can’t/couldn’t + be for logical impossibility in the
present, and couldn’t have + past participle for the past: to hurry.
It can’t/couldn’t be Paul at the door. He’s in Japan. In the past, these two forms have different meanings:
He can’t/couldn’t have had lunch yet. It’s only 11:15. We didn’t need to hurry. (= We didn’t hurry because
there was no need.)
4FF BMTP QBHF Expressing possibility, probability and We needn’t have hurried. (= We hurried but it wasn’t
certainty, for other modals and structures to express possibility.
necessary.)

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