can – could – to be able to

The verb can has two forms: can for the present tense and could for the past tense. Both can and could are followed by a bare infinitive.

Interrogative: Can I...? Could I ..., etc.

Negative interrogative: Can I not ...? Can’t I ...?, etc.

Other forms are supplied by allow, be allowed.

The negative forms of verbs are can’t and cannot (they mean the opposite of can).

can	can’t / cannot	I can’t sleep.
could	could not / couldn’t	We couldn’t sleep.
be able to	not be able to be unable to	She was not able to sleep. She was unable to sleep.

The form can is for all persons in the present and future, could is for past and conditional. All other missing tenses are supplied by the construction be able to which has all tenses.

Some people can ski.

I could work for 20 hours a day.

I could have done it yesterday if I had had time.

I’ll be able to translate this book next year.

NOTE

could you is pronounced with a [] sound. Could you pass me that newspaper? can is pronounced [] when it is stressed, usually in short sentences. – Can you come? – Yes, I 'can. can is pronounced [] when it is unstressed, usually in statements. I can 'speak Greek. can’t is usually stressed, and pronounced [:  :] (normally used in speech). cannot, more formal, is pronounced [   :]. I 'can’t swim. I cannot 'swim.

can / could / to be able to are used to talk about ability, awareness, and the possibility of something being the case. They are also used to say that someone has permission to do something. can or to be able to are used to talk about ability in the present. to be able to is more formal than can. In conditional sentences would be able to can be replaced by could.

could is often used in report structures.

He says / said ‘I can do it.’  He said (that) he could do it.

to be able to is sometimes used after modals such as might / should and after verbs such as want / hope / expect.

I might be able to help you

He might be able to hire a car.

I hope to be able to discuss it later..

You’re foolish to expect to be able to do it.

to be able to is used after used to, to say that someone had the ability to do something in the past but no loner does.

I used to be able to play the violin.

to be able to is used after will to talk about future ability.

After a few lessons, you will be able to understand basic German.

can / could also occur in adverbial clauses of purpose. This is the structurally dependent use.

I put the book on the table so that he can / could read it in peace.

The modals can / could have the following meanings.

1. can / could / to be able to are used to express ability.

can + verb is used to express natural ability; to be able to is less common. cannot or can’t are used to describe a lack of ability in the present. If the present ability is surprising or involves some difficulty, we can also use be able to. Another form for not be able to is be unable to (more common in formal English). to be able to / manage to / succeed in + -ing express ability in other tense combinations.

Can you run? (= Are you able to run? Are you capable of running?)

Despite his handicap he is able to drive a car.

The lawyer was unable to persuade the jury of his client’s innocence.

If you want to emphasise your ability to help, you can make an offer using can in a declarative sentence.

I have a car. I can drop Daisy off on my way home.

I can pop in at the shop tomorrow.

To describe a future ability only will be able to is used.

Will I be able to speak fluently by the end of the course?  Can I speak fluently by the end of the course?

will be able to is also used to make predictions.

By 2050 people will be able to learn a language by taking a pill.

can is used to express learned ability or ‘know-how’:

– Can you drive? – Yes, I can.

could / was able to / were be able to / used to are used to express past ability (natural and learned):

Could you drive last year?

Mozart could play the piano at the age of five.

NOTE

To describe the successful use of an ability on a specific occasion could is not used. Instead a past form of was / were able to is used. Mike’s car broke down but fortunately he was able to repair it. could can be used in questions and in sentences with limiting adverbs such as only and hardly. She was so exhausted she could hardly speak.

couldn’t or wasn’t / weren’t able to are used to describe a lack of ability or success, esp. when the action didn’t take place.

Mozart couldn’t speak French.  Mozart couldn’t / wasn’t able to understand French.

NOTE	can / could cannot be used to say that someone or something will have a particular ability in the future. Instead use be able to or be possible to.

would have been able to is used to express conditional past ability.

I wouldn’t have been able to find her in that crowd even if I’d known she was there.

could have (done) means would have been able to.

We could have gone away if we’d had enough money.

could / was able to are used to express achievement after effort:

I tried again and found I could swim / was able to swim.

could cannot normally be used to express specific achievement in the past when we are describing the successful completion of a specific action. was / were be able to / managed to / succeeded in + -ing must be used instead.

In the end they	were able to	rescue	the cat on the roof.
In the end they	managed to	rescue	the cat on the roof.
In the end they	succeeded in	rescuing	the cat on the roof.

If the action was not realised in the past it is expressed with the help of to fail, or to manage or to succeed in the negative form.

He failed / didn’t manage to swim across the river last summer. (Он не мог переплыть реку в прошлом году. Ему не удалось переплыть реку в прошлом году.)  He could / managed to swim across the river last summer. (Он мог переплыть реку в прошлом году.)

If an action was not successfully completed, we may use couldn’t. could can be used when we are asking about a specific action:

– Could they rescue the cat on the roof? – No, they couldn’t. It was too difficult.

An affirmative response requires an alternative to could.

– Could they rescue the cat on the roof? – Yes, they managed to.

can / could can also express physical ability or awareness. As verbs of perception rarely occur in the progressive, can, and to a lesser extent, to be able to, combine with such verbs (see, hear, smell, taste, etc.) to indicate that we can see, hear, etc. something happening at the moment of speaking:

I can’t see anyone. = I don’t see anyone.

I can hear footsteps; who’s coming?

I can smell it. Can’t you?

can / could can be used with verbs suggesting ‘ understanding’:

I can’t understand it. I can’t realise it.

I couldn’t understand what was happening.

NOTE

1. could + infinitive and could have + past participle are used to express hypothetical ability: You could do it if you tried. You could have done it yesterday if you have tried. 2. could have / might have + past participle are used: to describe a past ability which wasn’t used: She could have paid by credit card but she preferred to use cash. (= She had the ability to pay by credit card but she didn’t use it.) to describe a past opportunity which wasn’t taken (the third conditional): I might have gone to university after leaving school but I began to work. (I had the opportunity to go, but I didn’t take it.) to make a criticism: You might have told me about the party! You could have told me about the party! (= You had the chance to tell me but you didn’t.) 3. could have + past participle + comparative adjective is used to emphasise a past action or feeling: They couldn’t have tried harder to make me feel welcome. (= They tried very hard.)

2. can / could ‘иметь возможность, быть в состоянии’ are used to describe things which are generally possible, i.e. to speak about possibility (a) due to the existing laws or (b) due to circumstances. can expressing possibility is used with the indefinite infinitive only and it refers the action to the present or future. In past-time contexts could is used. In present-time contexts could + perfect infinitive expresses unreality with reference to the present or future.

Drinks in restaurants can be very expensive. (= Drinks in restaurants are sometimes very expensive.)

She could have explained the mystery.

Subject + can / could (for the past) means ‘it is possible’, i.e. circumstances permit (general or occasional possibility).

You can ski on the hills. (= There is enough snow.)

Measles can be quite dangerous.

NOTE

In scientific and academic English may / might are used in the same way. Antibiotics may lead to the best effect. To talk about specific possibilities may / might and could (not can) are used. The meaning is similar to ‘perhaps’ or ‘maybe’. There can be life on Mars.  There may be life on Mars. (= Perhaps there is life on Mars.)

To describe possibilities which depend on certain conditions could / might are used.

She could learn much more quickly if she paid attention.

well can be used after could / may / might if we think the possibility is quite strong. If we are very certain of the possibility, we use must.

Don’t worry, the contract could well be in the post. (= It is probably in the post.)