The modal verb can is used with a bare infinitive to express the idea that something is possible or that someone is able to do something or allowed to do something.
Could is the past tense of can, but is also used in a number of other ways.
| Present Tense | Past Tense |
| I know I can help them. They say they can’t come. ‘I can help you,’ he said. ‘We can come back tomorrow,’ I told them. ‘Can I have another biscuit?’ he asked. | I knew I could help them. They said they couldn’t come. He said he could help us. I told them we could come back the next day. He asked if he could have another biscuit. |
Describing Possibilities
Can is used to talk about what is possible or what someone or something is able to do.
- Can is used to talk about what is possible or what someone or something is able to do.
- You can see for miles from the top of that hill.
- Nowadays you can use online search facilities to find the best bargains.
- Elephants can run surprisingly quickly.
- You can no doubt understand how upset I was when I heard the news.
- Can a python really swallow a whole antelope?
- I simply cannot answer all these questions in the time we have left.
- Ostriches are birds but they can’t fly.
- We’d like a new car but we can’t afford one.
- I can’t reach the packets of soup on the top shelf. Can you reach them?
In this sense of can, the past tense is could:
- From the top of the hill, we could see for miles.
- Could mammoths run as fast as elephants can now?
- Like ostriches, the moas that once lived in New Zealand couldn’t fly.
- When I was young, we couldn’t afford a car.
- It’s a pity they couldn’t come with us yesterday.
Can and could are often used with verbs that describe the use of the mind or the senses (sight, hearing, touch, etc), but without much idea of ‘possibility’ or ‘ability’ at all:
- I can see you. (= I see you)
- We know that the lions can hear us from the way they are looking in this direction. (= … the lions are hearing us …)
- I can’t remember where I put the money. (= I don’t remember where I put it)
- I could feel something crawling up my leg. (= I felt something crawling up my leg)
- We could see he wasn’t happy. (= We saw that he wasn’t happy)
- When I got to the shop, I couldn’t remember what I had wanted to buy. (= I didn’t remember what I had wanted to buy)
- I couldn’t understand a thing she said. (= I didn’t understand her)
Couldn’t is used when politely refusing an offer of something to eat or drink:
- ‘Would you like another piece of cake?’
- ‘Oh, I couldn’t eat another thing.’
Couldn’t and couldn’t have are used as a way of simply emphasizing what you are saying:
- I couldn’t agree with you more. (= I agree with you completely)
- They couldn’t have been more surprised to find they had won the prize.
When you are talking about one single event or action in the past, you can only use could along with a negative word or with one of the verbs of hearing, seeing, etc just mentioned in the paragraph above:
- I couldn’t get the cork out of the bottle.
- When he threw the ball to me, I couldn’t catch it.
- No-one could solve the puzzle.
- She was so upset, she could hardly speak.
- I could see she was upset.
Otherwise, when talking of a single event in the past, you have to use be able to or a verb such as manage or, in more formal English, succeed:
- Were you able to speak to Jack last night?
- I managed to get the cork out of the bottle.
- He succeeded in catching the ball when she threw it to him.
Could without a negative word describes a general situation or condition in the past:
- When we were young, we could play in the woods all day long.
- We didn’t have a phone in our house but we could use the telephone box at the end of the street.
Could is used to refer to less certain or more doubtful possibilities in the present or the future.
- Could a python really swallow a whole antelope? (= would it be possible for a python to eat an antelope?)
- You’d better take your umbrella with you. It could rain later on. (= it might rain)
- The parcel could arrive tomorrow.
- That could be the explanation for his strange behaviour.
When referring in this way to more doubtful possibilities, you use could have when referring to the past:
- Could a python really have swallowed a whole antelope?
- I suppose a bird could have stolen the ring but I rather doubt it.
- The police are not sure what was used to kill her. It could have been a pipe or an axe handle.
Could is also used to refer to something that is possible in theory but which does not actually happen or is not actually done:
- The police could do more to prevent riots, (but they don’t)
- A lot of crime could be prevented if people took more care.
When referring to the past, you again use could have:
- I suppose I could have gone with them, but I didn’t really want to. (so I didn’t go)
- We could have stayed another day, but we decided not to.
- Could this tragic accident have been prevented? (it wasn’t prevented)
Can and could may both be used when you are wondering about something.
- Who can that be knocking on our door so late at night?
- I wonder who that could be, knocking on our door so late at night.
- What on earth can have happened to him?
- Where on earth could he be at this time of night?
In this sense, there is little difference between can and could. The past tense is could have:
- Who could have been knocking on my door so late in the night?
Could is used to describe something a person wants to do, but which they may or may not do; could have is used to describe what a person wanted to do or felt like doing, but didn’t do.
- I feel so angry, I could just stamp my feet and scream.
- I felt so angry, I could have hit him.
Sometimes cannot, can’t and couldn’t are used to express the opinion or hope that something is not possible or not true.
- That story in the papers can’t be true, surely.
- What she told you couldn’t be true.
- The conflict in the Middle East can’t last forever.
- The town council surely can’t be expecting us to pay for the repairs to the pavement.
- They can’t be out. I can hear a radio playing in the front room.
Couldn’t have is used to refer to the past:
- They couldn’t have been out. I could hear a radio playing in the front room.
Can’t, couldn’t, can’t have, etc may express surprise or disbelief or disapproval:
- But we can’t have spent all our money yet! We should have at least another $200.
- How could we have spent all that money in Just two days?
- That can’t be your parents here already! They’re not due to arrive until this evening.
- She couldn’t have been one of the terrorists!
The same is true of question forms:
- How could you have been so stupid? OR
- How could you be so stupid?
Describing Knowledge or Skills
Can is used to say that someone knows howto do something.
- Her brother can speak German.
- My little daughter is only three but she can already write her name.
- Help! I can’t swim!
- Some children still cannot read by the time they leave school.
In this sense, the past tense is could:
- I could speak French when I left school but I can’t any more.
- I never could speak French.
- He could compose music by the age of four.
Referring to What Is Permitted
Can is used to ask for or give permission or to say that something is allowed.
- ‘Can I borrow your bike? ‘No, you can’t.’
- Can we ask questions at the meeting?
- We can take books with us into the exam room.
- European Union regulations now tell us what fish we can and cannot catch.
When describing a general situation in the past, the past tense is could:
- We could take dictionaries with us into language exams if we wanted to.
But when describing a single event or occasion in the past, you have to use some other word or phrase, such as be allowed to or be permitted to:
- As a special concession, she was allowed to take her dog with her.
In the negative, the past tense is could not or couldn’t.
- We couldn’t take books with us into the exam room when I was your age.
- Unfortunately she could not take her dog onto the plane with her.
Could is also used to ask permission. It is slightly more polite than can:
- Could I possibly borrow your car?
- If I could just interrupt you for a moment, there is something I would like to say.
Note that although you can use could to ask for permission, you use can when giving permission:
- ‘Could I possibly borrow your car?’ ‘Yes, of course you can.’
Making Requests and Suggestions
Can is used to make a request or to invite someone to do something.
- Can you tell me what time it is?
- Can you help me, please? I’m afraid I’m lost.
- Now, if you can just wait here a moment, I’ll see whether the manager is free to speak to you.
Requests formed with could are slightly more polite:
- Could you move along a bit? I don’t have much room.
- You couldn’t move along a bit, could you?
- Excuse me. Could you give me a hand to push my car? It’s broken down.
- Could I open a window, please? It’s very hot in here.
Requests formed with can and could may also show some annoyance or impatience:
- Can you make a little less noise, please!
- Could you hurry up there!
In negative questions, can’t also expresses impatient or angry requests or complaints:
- Can’t you be a little quieter when you’re playing? I’m trying to get some sleep.
- Why can’t men be as sensible as women?
Could is used to make suggestions:
- If the car won’t start, you could try pushing it along the road.
- You could always ask your father to lend us the money.
Sometimes could and could have are used to make suggestions that show that the speaker is annoyed:
- You could be more of a help to me, you know!
- Well, you could have told me you weren’t coming!
Other Uses of Can and Could
Can may be used to describe something that sometimes or frequently happens:
- She can be a little rude at times but she doesn’t mean it.
- It can get very cold here in winter.
- Schools in London can have pupils from many different ethnic backgrounds.
In this sense, the past tense is could:
- She could be very rude at times but then at other times she could be very kind.
Cannot and can’t are used to suggest that something should not happen:
- We can’t leave yet. We’ve only been here five minutes.
- We can’t just walk out in the middle of the meal, can we? We’ll have to stay till the end.
Both couldn’t and couldn’t have are used to refer to the past:
- We couldn’t leave after only five minutes.
- We couldn’t have left after only five minutes.
Can and could are used to express willingness:
- Yes, I can come back again tomorrow if you want me to.
- I could come back tomorrow if that is more convenient.
- The management cannot accept any responsibility for damage to customers’ cars left in the car park.
- It’s kind of you to offer to pay for my trip, but I’т afraid I can’t accept your help.
- It’s very kind of him but I just couldn’t accept his offer.
Could and could have are used to say that something is like something else or has the appearance of something else:
- The sea is so smooth, it could be glass. (= it looks like glass)
- From the top of the hill, the cars looked so small that they could have been little insects.
Can is sometimes used to express an order.
- You can stop that nonsense right now!
- Since you can’t behave yourself, you can just go home.
Note the tenses used in conditional sentences:
- If the old clubhouse falls down, we can build a new one.
- If the old clubhouse fell down, we could build a new one.
- If the old clubhouse had fallen down, we could have built a new one.