Can or could?
Like may and might, can and could cause a lot of confusion.
Understanding how all the modal verbs are used is vital to speaking and writing English effectively and idiomatically, so let’s explore the meanings and uses of can and could. Given that these are quite complex verbs, I’d like to focus on explaining some key points (otherwise this post would be verging on a book chapter, lengthwise). If you’re interested in exploring other issues, I recommend consulting a reference book, such as Practical English Usage by Michael Swan, which is written for learners of English but which is also useful for native English speakers who don’t have specialist grammatical knowledge.
Can and could
The main ways in which the verb can is used are given below. In some cases, could functions as the past tense of can, but there are some important usage differences between the two.
1.) Can is used to say that someone or something is able to do something, either now, or as a natural characteristic, as a continuing skill, as something learnt:
After five operations, today he can walk and run.
A sea snake can live its whole life without ever touching land.
Can you speak Greek?
- We tend to use could as the past tense of can to talk about ability in the past:
I could hear Beth sniggering and cringed in embarrassment.
Mozart could play the piano blindfolded.
By the 1970s, jumbo jets could fly almost anywhere non-stop.
2.) When talking about what is possible in a given situation, or about an opportunity that is open to someone, use can or be able to:
By joining a club or gym, you can meet so many people.
Parents can save money by booking their holiday outside the peak season.
I can go to London tomorrow as I’ve got the day off.
- We use could to talk about less definite aspects of possibility or suggested options, either now or in the future:
We could go for a walk now and then have dinner.
There could be a storm later.
She could be in line for a top government job.
It would be acceptable to use can in the first example, where a suggestion is being discussed (We can go for a walk now…), but this would make the suggestion or option more definite. It’s not idiomatic English to use can in the second and third examples though, which refer to a hypothetical future situation which, although possible, may well not happen:
X There can be a storm later.
X She can be in line for a top government job.[It’s also possible to use may and might here, rather than could.]
- We use could have to refer to something that was an option or generally possible in the past but didn’t happen:
She never stopped daydreaming about the life she could have lived in Greece.
3.) Can is used to talk about being allowed to do something now or in general:
Resorts can only be built on deserted islands, and must have their own generators.
You can have an hour for lunch, except on Fridays.
- If you’re referring to a general past situation when something was allowed, use could:
The Americans were under instructions that no-one could smoke indoors.
But if something was only permitted on a particular past occasion, it is more common to use a different wording rather than could, for example:
√ I was allowed to leave work early today as I had to go to the dentist.
X I could leave work early today as I had to go to the dentist.
- See the section on Can and could versus may and might below with regard to asking for permission, making requests, etc.
So far, we’ve seen that could is often used as the past tense of can. Other important meanings and uses include the following.
- Use could (not can) to refer to conditional situations, in which something has to happen or be the case in order for someone to be able to do something or for something else to occur:
We could buy a new sofa if we stop eating takeaway meals every night.
- People often use could (never can) to talk about completely unrealistic situations, so as to deliberately exaggerate how they’re feeling at a particular time:
√ I’m so angry, I could murder her.
X I’m so angry, I can murder her.
To refer to a past unrealistic situation or strong inclination, use could have:
She was so thirsty, she could have drunk a gallon of water.
He irritated me so much that I could have screamed.
- Could have is also used (in a similar way to might) to show annoyance when you think someone should have done something, but they didn’t:
You could have told me that she wouldn’t be at work today!
Can or could versus may or might
This section provides more information on some points outlined in the may and might post, concentrating on the way these verbs are used to make offers and requests and to ask for and give permission.
1.) Requests and offers
- When making a request for something, the most usual way to do this in everyday English is to use can or could:
Can I have two coffees please?
Could I have two coffees please? [more polite than can]
Although can and could are perfectly acceptable, some people prefer to use may in such cases, as it’s regarded as more polite and more formal:
May I have two coffees please?
Nowadays, using might to make requests is generally reserved for very formal situations and to make the request sound more like a polite suggestion than a firm instruction:
Might I ask the Court to glance briefly at the judgment of Sir Harry Gibbs?
- When making an offer, can is the most frequent way of doing this in everyday English; could is used when we want the offer to sound more tentative; may is more formal and more polite:
Can I get you another drink?
Could I help you in any way?
May I get you another drink?
2.) Asking for and giving/refusing permission
- The most typical way of asking for permission in today’s English is to use can, or if you want to sound more polite, could:
Can I borrow your pen?
Could I borrow your pen?
Although this is part of standard English, many people believe that can and could are incorrect within the context of permission and should be reserved for talking only about ability and possibility, and thus it is advisable to use may in more formal writing and speaking (might is regarded as very formal):
May I borrow your pen? [polite, formal]
Might I borrow your pen? [rare, polite, very formal]
- When giving (or refusing) permission, only can (or can’t) and may (or may not) are acceptable, can being subject to the same caveats as when asking permission to do something:
√ Yes, you can (borrow my pen) [everyday English, considered incorrect by some]
√ Yes, you may (borrow my pen). [more polite/formal]
X Yes, you could (borrow my pen).
X Yes, you might (borrow my pen).
√ No, you can’t (borrow my pen). [everyday English, considered incorrect by some]
√ No, you may not (borrow my pen). [more polite/formal]
X No, you could not (borrow my pen).
X No, you might not (borrow my pen).
Incidentally, can, could, and might would be acceptable answers if you wanted to express possibility or conditionality rather than give permission. For example: yes, you could borrow my pen would imply that yes, if I had a pen, it would be possible for you to borrow it.
That’s all for now: I hope this has clarified some aspects of can and could. More on modals in future blogs!