What is the Difference Between Can and Could?
The difference between can and could is one of usage more than definition. You may have heard the difference being explained as could being the past participle of the present tense can, but this is only part of their subtle yet complex differences. The main reason for this is due to them being modal verbs. Modal verbs are verbs which are used as an auxiliary verb to the main verb. They provide information about the main verb of the sentence, specifically about either the possibility or necessity of the action it describes. If this seems a little confusing, it's because it is. Many English speakers know when to use can or could simply because they have practice and experience using them. oneHOWTO further explores what is the difference between can and could so that we might best know when to use them.
When is 'can' used?
As can is a modal verb, it helps provide greater meaning for the main verb in terms of what is possible or what is necessary. This is a spectrum which can be broken up into 4 parts:
- Likelihood/possibility of occurrence
- Ability of occurrence
- Permission to occur
- Obligation to occur
Other modal verbs used in the English language include may/might, shall/should, will/would and have to/must. Both can and could are used in all of these 4 types of modal uses, except for obligation.
When indicating permission, possibility or ability and using the present tense, can is used:
- Can I go to the zoo? (Permission)
- You can buy one of these things. (Possibility)
- John can play the piano. (John has the ability to play the piano)
The negative of can is cannot, although it can be contracted to can't. The meaning is logical; it indicates not having permission, something being impossible or not having an ability:
- Mom says we cannot go to the zoo today. (Not having permission)
- The bus can't have arrived yet. (Impossibility)
- Eva can't swim. (Eva does not have the ability to swim)
If you say can not with the words separared, it means something different. Instead of not having permission, possibility or ability, you're saying that you have the permission, possibility or ability of not doing something:
- We can not go to school today. (We have permission to skip school)
- Peter can not breathe for a long time. (Peter has the ability to hold his breath for a long time)
When is 'could' used?
Similarly, could is a modal verb used as an auxiliary verb to the main verb for permission, possibility or ability. The difference here is that could is used for the past tense. It is used especially if the sentence no longer applies to the present:
- You could have come to the party. (You had permission in the past [but not longer])
- He could have been famous. (He had the possibility in the past [but no longer])
- Mary could speak German. (She had the ability in the past [but no longer])
This last example is an interesting one because it refers to an action which would normally continue into the present. If the sentence 'Mary could speak German' is used rather than 'Mary can speak German', it implies that she has either forgotten how to speak a language or that she is unable to speak it anymore because she is no longer with us.
The negative of could is could not or couldn't and it is used just like it was in can.
- I couldn't go out yesterday. (I didn't have permission)
- She couldn't afford going on holiday. (It was an impossiblity)
- Peter could not swim as a child. (He didn't have the ability)
It has even more uses. Could is also used as an auxiliary verb to make polite requests.
- Could you buy some sugar, please?
- Could you please pass the salt?
It is also a conditional for capacity and skill:
- You could join us if you wanted to.
- I could write a book if I had time.
Conditional sentences are those with two 'clauses', one clause determining the other.
'Can', 'could' and 'may'
When making requests, asking permission or stating possibility, there is a certain amount of interchangeability with can and could, but the word may also comes into play. This interchangeability is seen when making requests:
- Can you read my first draft?
- Could you read my first draft?
There difference here is one of formality. Could is a little more formal and often used when you want to request something from someone you wish to be deferential towards. It is also considered somewhat more polite.
If asking permission from someone, you use can and could, but there are times when it is more suitable to use the modal verb may. Examples include:
- Can I borrow a towel? (informal)
- Could I borrow a towel? (less formal)
- May I borrow a towel? (formal/polite)
You can often use the modal verb might instead of may as they are similarly interchangeable. This is a difference which many teachers point out when speaking to pupils. This can be highlighted in a short dialogue:
- Pupil: 'Can I go to the toilet?'
- Teacher: 'You could go to the toilet, but I won't let you.' (pupil has the ability to go, but not the permission)
- Pupil: 'May I go to the toilet?'
- Teacher: 'Yes, you may go.'
Another use of these three modal verbs is in possibility of an occurrence and they are bet shown by example:
- It can be hard to grow up here.
This is a generally perceived truth, but also implying there is a strong possibility it is true. It is not absolute. If it were, you would say 'It is hard to grow up here.'
- It could be hard to grow up here.
This means that while it is hard for some people to grow up here, it is not the same for everybody. It would often be used with a conditional, e.g. 'It could be hard to grow up here, if you don't have much money.'
- It may be hard to grow up here.
This is very similar to using could, but it also implies that it is less likely. Can also be used with a conditional, e.g. 'It may be hard to grow up here, but only if you're unlucky.'
It is tricky to know when to use can and when to use could as they are often, but not always interchangeable. Many people use one or the other in paragraphs simply because they have used the other before.
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