can - could - be able to

can - could - be able to
These words 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. These uses are dealt with separately in this entry.
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Can and could are called modals. See entry at ↑ Modals.
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Both can and could are followed by an infinitive without `to'.

Some people can ski better than others.

I could work for twelve hours a day.

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negative forms
The negative form of `can' is cannot or can't. Cannot is never written `can not'. The negative form of `could' is could not or couldn't. To form the negative of `be able to', you either put `not' or another negative word in front of `able', or you use the expression be unable to.

Many elderly people cannot afford telephones.

My wife can't sew.

It was so black you could not see a hand in front of your face.

They couldn't sleep.

We were not able to give any answers.

We were unable to afford the entrance fee.

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ability: the present
Can, could, and be able to are all used to talk about a person's ability to do something.
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You use can or be able to to talk about ability in the present. Be able to is more formal than can.

You can all read and write.

The goliath frog is able to jump three metres or so.

...people who are unable to appreciate new ideas.

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Could is also used to talk about ability in the present, but it has a special meaning. If you say that someone could do something, you mean that they have the ability to do it, but they do not in fact do it.

We could do a great deal more in this country to educate people.

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ability: the past
You use could or a past form of be able to to talk about ability in the past.

He could run faster than anyone else.

A lot of them couldn't read or write.

I wasn't able to do these quizzes.

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If you say that someone was able to do something, you usually mean that they had the ability to do it and they did it. Could does not have this meaning.

After treatment he was able to return to work.

The farmers were able to pay the new wages.

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If you want to say that someone had the ability to do something but did not in fact do it, you say that they could have done it.

You could have given it all to me.

You could have been a little bit tidier.

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If you want to say that someone did not do something because they did not have the ability to do it, you say that they could not have done it.

I couldn't have gone with you, because I was in London at the time.

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If you want to say that someone had the ability to do something in the past, although they do not now have this ability, you say that they used to be able to do it.

I used to be able to make it happen.

You used to be able to see the house from here.

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ability: the future
You use a future form of be able to to talk about ability in the future.

I shall be able to answer that question tomorrow.

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ability: report structures
Could is often used in report structures. For example, if a man says `I can speak Arabic', you usually report this as `He said he could speak Arabic'.

Ferguson said I could ask for a transfer if after six months I still don't like it.

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ability: 'be able to' after other verbs
Be able to is sometimes used after modals such as `might' or `should', and after verbs such as `want', `hope', or `expect'.

I might be able to help you.

You may be able to get extra money.

You should be able to feel this.

She would not be able to drive to inland cities alone here.

You're foolish to expect to be able to do that.

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You do not use can or could after any other verbs.
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'being able to'
You can use an `-ing' form of be able to.

...the satisfaction of being able to do the job.

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There is no `-ing' form of can or could.
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Can and could are used with verbs such as `see', `hear', and `smell' to say that someone is or was aware of something through one of their senses.

I can smell gas.

I can't see her.

I could see a few stars in the sky.

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Note that this is the most common way of expressing awareness through one of your senses. For example, if you become aware of a phone ringing, you say `I can hear a phone ringing'. You do not say `I hear a phone ringing'.
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possibility: the present and the future
Could and can are used to talk about possibility in the present or future.
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You use could to say that there is a possibility that something is or will be the case.

Don't eat it. It could be a toadstool.

There could be something in the blood.

300,000 jobs could be lost.

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Might and may can be used in a similar way.

It might be a trap.

Kathy's career may be ruined.

See entry at ↑ might - may.
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You do not use could not to say that there is a possibility that something is not the case. Instead you use might not or may not.

It might not be possible.

It may not be easy.

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If you want to say that it is impossible that something is the case, you use cannot or could not.

Kissinger cannot know what the situation is in the country.

You can't talk to the dead.

It couldn't possibly be poison.

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You use can to say that something is sometimes possible.

Such shifts in opinion can sometimes have a snowball effect.

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possibility: the past
You use could have to say that there is a possibility that something was the case in the past.

He could have been doing research on his own.

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Might have and may have can be used in a similar way.

The teacher might have known the local policeman.

It may have been a dead bird.

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You also use could have to say that there was a possibility of something being the case in the past, although it was not in fact the case.

It could have been worse.

I could have escaped sentence by inventing a false name for my informant.

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You do not use could not have to say that there is a possibility that something was not the case. Instead you use might not have or may not have.

She mightn't have known what the bottle contained.

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If you want to say that it is impossible that something was the case, you use could not have.

I couldn't have known that in a few weeks I would lose control too.

The man couldn't have thought at all.

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Can and could are used to say that someone is allowed to do something.

You can take out money at any branch of your own bank.

He could come and build in my wood.

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Cannot and could not are used to say that someone is or was forbidden to do something.

You can't bring outsiders into a place like this.

`May I speak to Mr Jordache, please?' —-`No, you can't.'

Standish could not have questioned the man; outside a Customs enclosure a Customs officer had no right to interrogate anyone.

See also entry at ↑ Permission.
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Useful english dictionary. 2012.

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  • can — can1 W1S1 [kən strong kæn] modal v negative short form can t ▬▬▬▬▬▬▬ 1¦(ability)¦ 2¦(requesting)¦ 3¦(allowed)¦ 4¦(possibility)¦ 5¦(seeing/hearing etc)¦ 6¦(not true)¦ 7¦(should not)¦ 8¦(surprise/anger)¦ 9¦(sometimes)¦ …   Dictionary of contemporary English

  • can — [n1] container, usually metallic aluminum, bottle, bucket, canister, cannikin, gunboat*, gutbucket*, jar, package, pop top*, receptacle, tin, vessel; concepts 476,494 can [n2] toilet head*, john*, johnny*, latrine, lavatory, litter box*, outhouse …   New thesaurus

  • can — 1 strong verb (modal verb) 1 to be able to: He s so tall he can touch the ceiling. | This machine can perform two million calculations per second. | I can t remember where I put it. | They have everything that money can buy. | The police still… …   Longman dictionary of contemporary English

  • could — strong /kUd/ modal verb 3rd person singular couldnegative short form couldn t 1 the past tense of can : Could you hear that all right? | I couldn t get tickets after all, they were sold out. | Marcia said we could smoke, it was okay with her. see …   Longman dictionary of contemporary English

  • Could — Can Can, v. t. & i. Note: [The transitive use is obsolete.] [imp. {Could}.] [OE. cunnen, cannen (1st sing. pres. I can), to know, know how, be able, AS. cunnan, 1st sing. pres. ic cann or can, pl. cunnon, 1st sing. imp. c[=u][eth]e (for… …   The Collaborative International Dictionary of English

  • Can — Can, v. t. & i. Note: [The transitive use is obsolete.] [imp. {Could}.] [OE. cunnen, cannen (1st sing. pres. I can), to know, know how, be able, AS. cunnan, 1st sing. pres. ic cann or can, pl. cunnon, 1st sing. imp. c[=u][eth]e (for cun[eth]e); p …   The Collaborative International Dictionary of English

  • can — can1 [kan; , kən] v.aux. pt.could [ME < OE, 1st & 3d pers. sing., pres. indic., of cunnan, know, have power to, be able; common Gmc < IE base * gen , *gno > L gnoscere, KNOW; orig. meaning “to be able mentally or spiritually,” as… …   English World dictionary

  • can — noun. Can is the word generally used in BrE for the container when the contents are liquid (a can of beer / a can of soup). When the contents are solid, tin is more usual (a tin of beans / a tin of peaches) but can is used for this too in AmE.… …   Modern English usage

  • could — modal auxiliary. 1. See can. It functions as (1) the past tense of can, as in We could see for miles, (2) as a conditional equivalent to would be able to, as in I could take you in the car if you like, and (3) as a more tentative form of can in… …   Modern English usage

  • able to — The construction to be able to (do something), with an active to infinitive, is a natural part of the language, extending to inanimate as well as animate subjects • (By his proceeding to the beach…the next phase of the attack was able to proceed… …   Modern English usage

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