In the English language, the modal verb pair can/could is one of the most difficult modal pairs to learn as the modals can be used in a variety of situations. These include:
- to show ability
- Birds can usually fly
- to make an offer
- I can do that for you.
- to ask permission
- Can I borrow a pen?
- to talk about possibility/impossibility
- Humans can’t fly.
- to make a request
- Can you move your car?
- to make a suggestion
- We can meet tomorrow.
The difficulties that many English-learning beginners have is that can and could are used in many situations and can even be used in the same situations. When you throw in could have and couldn’t have, things start to get complicated! Choosing one over the other modifies the meaning, either slightly or radically. But don’t worry, this explanation should help =)
Ability + can/could
Both can and could can be used to show the ability of something. For example:
- Most birds can fly. – ability in the present
- Some dinosaurs could fly. – ability in the past
Note: Often in the past, the verb was/were able to is used instead of could. The difference is that, if you are making a general statement about the past, English speakers use could. If the statement is not general, was/were able to is used instead:
- I was able to go to the bank yesterday despite the Covid-19 lockdown. – specific ability
Note: In the future, this change also happens, becoming will be able:
- Cats will be able to fly in the future.
- They could come to the party by car. – possibility but not certain
This also includes things that were possible in the past but did not happen, such as:
- I could have been a millionaire if I had started saving money earlier.
Note that for this possibility in the past that did not happen, we use could have instead of just could.
Offers + can/could
This modal pair can also be used to make offers. For making offers, the relationship between can and could is one of politeness. Can is more informal while could is more polite. For example:
- Can I help you?
- Can you say thanks to your mum for me? (familiar)
You can also use could:
- Could you say thanks to your mum for me? (more polite)
Permission + can/could
While many people believe that can/could should not be used to ask for permission (see May/Might for permission), there are people who use can instead. For example:
- Can I go to the loo? – May I go to the loo (asking permission)
- Could we go there tomorrow if we have time? – using could in a conditional sentence to make it more polite. This form is becoming less common
Possibility/Impossibility + can/could
One of the most common ways that can and could are used in English are to talk about the possibility or impossibility of something. For example:
- I can come over tonight if you are free. – possibility in the near future
- I can do it now if you need it soon. – possibility in the present
You can also talk about things that are possible but you are not certain whether they will happen. For example:
Requests + can/could
For requests, the difference between can and could is how polite you intend to be. For example, let’s take the following sentence:
Typically when using can for questions that make a request, the question is considered informal. It would be fine if you were to say this to a friend but would be rude if you were to say it to a stranger. You can make this sentence more formal using ‘could’:
- Could you move your car?
So what happens if you accidentally use can instead of could? Well it depends on the person to whom you are talking. Some people might take offence, finding your statement aggressive, while others will not mind. Your tone of voice is also very important; how your tone comes across can change the meaning.
- Can you move your car?
Suggestions + can/could
When making suggestions, could is the modal verb of the pair that is used most often when making a suggestion (although can can be used, it is not common in most cases):
- What should we do for our date? We could go see a film and then have dinner.
But what about “couldn’t have”?
When we use a modal verb followed by the present perfect tense, we are speculating about the past. This is also true of can and could but there is an exception. It is not possible to use can with the perfect tense. Only could can be used in the positive, such as could have gone. When could have is used, the sentence shows a possibility in the past that did not happen. For example:
- It could have fallen through the gap if you had dropped it.
When used in the negative sense, you can combine both can and could with the present perfect tense. When used as can’t have and could’t have, the user makes a negative logical conclusion in the past. For example:
- She is here right now so she can’t have gone/couldn’t have gone home.
- She is here right now so she couldn’t have gone/couldn’t have gone home.
These forms are generally interchangeable in most situations.
In summary (can vs could):
|Possibility / Impossibility||Certain Possibilities |
– I can do it now if you want.
– Birds can fly.
– Humans can’t fly
Impossible in past
– The can’t have got lost.
|Possible but not certain|
– They could come by car.
General Statements about past
– Some dinaosarus could fly.
Guesses about the past
– They could have finished already.
Impossible in past
– They couldn’t have got lost.
– Birds can fly.
Do something in present or future
– You can dance now.
|Talk about past|
– She could walk ten kilometres.
Had the ability/opportunity but didn’t do something
– I could have climbed that mountain.
|Permission||Ask for something (less formal)|
– Can I borrow a pen?
– You can go now.
Already have permission
– Students can travel for free.
|Ask for something (more formal)|
– Could I borrow a pen?
|Requests||Less polite/rude request|
– Can you move?
|More polite request|
– Could you move?
|Offers||I can do that for you.||I could do that for you.|
|Suggestions||– We can meet tomorrow.||– We could meet tomorrow.|
Can and could also have a relationship with the verbs to be able to and to be allowed to. In the past tense, if you are talking about an ability during a specific event or an opportunity, you would use be able to:
- Due to the adrenaline rushing through his veins, Harry was able to lift a car! – ability
- As I had some free time yesterday, I was able to help her – opportunity
If you are talking about having or requesting permission in the past, you may use be allowed to:
- I was allowed to drive her car because her leg was broken. – permission
In the future, can is used for opportunities, permission, requests and possibilities. Why? Because these things usually talk about the near future, generalisations or suppositions (the last two use English present tense). However, if you are talking about ability or the far future, you would use will be able to:
- Can I come over tomorrow? – permission in the future
- When I get my letter from Hogwarts, I will be able to become real a wizard! – ability in the future
To be able to and to be allowed to are not modal verbs; the act like normal verbs and follow the normal rules or verbs.
Want to learn more about modal verbs? You can find a guide to all the modal verbs here: introduction to modal verbs. You can also learn more about other English language information on my English Language Materials page.