Can vs Could

The Uses of Can and Could

The modal verbsOpens in new window, can and could, are derives from the verb ‘to be able’, denoting ability. In this way, they are used to express people’s ability to do certain things, either in the present (can), or in the past (could).

A.   The Uses of Can

The modal auxiliary can is used in many ways. We can identify the uses of can under the following headings.

1.  Expression of Capability

Can is usually used to express ability, capability, fitness, capacity, skill in the sense of being able to do something or knowing how to do something. Survey the example below:

  • Andy can play a piano but he can't sing very well.

Andy's ability makes him able to play the piano but he doesn't have ability to sing very well. Observe also the following:

  • I can run a mile in under four minutes.
  • The weather can change so quickly in the mountains.
  • He can speak Spanish language fluently.

2.  Expression of Permission

We use can to express permission in the sense of being allowed or permitted to do something, as the example sentences below illustrate.

  • Can I ease myself in your rest room?
  • You can sleep in that room, if you want to.
  • He can go home, if he is tired.
  • The teacher says that we can play on the field.
  • You can go only after you have finished your work.
Important Hint!  

Note that can is less formal than may. Hence, some grammarians will disagree to the use of can in certain contexts. Note also that can’t sometimes expresses ‘prohibition’, as the following sentences show:

  • You can’t smoke here.
  • I can’t make the door open.

3.  Expression of Theoretical Possibility

Can is also used in the expression of theoretical or hypothetical possibility, impossibility, opportunity, and likelihood. Survey the following examples.

  • Can there be any doubt about his intentions?
  • Temperature can reach 40 degrees in the summer.
  • It can be cold at night.
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Important Hint!  

Note that some sentences in the context of hypothetical possibility have a condition clause and a clause that shows the result of that clause, for example, “If I can get home early enough”, (condition clause) “I can attend the party.” (result of the condition clause). Consider the following:

  • My trainer says I can win the race if I really work hard.
  • If it’s fine tomorrow, we can go fishing.

4.  Expression of Request

Can may also be used to express requests, as follows:

  • Can I offer you something to drink?
  • Can you help me?
  • Can I ask a question?
  • Can you help me solve this problem?
  • Can I make a suggestion?
Important Hint!  

Note that Can, Could and may are used in questions which are requests for permission. Could and may are considered to be a little more academic or formal and polite than can. However, permission is also given with can or may, as shown below:

  • Can I use your telephone? (informal)
  • Could I use your telephone? (more polite)
  • May I use your telephone? (more formal)

B.   The Uses of Could

The modal auxiliary could is used in multiple ways to express various shades of meaning. We can identify the uses of could under the following headings.

1.  Expression of Ability

Could is often used to express an ability in the past in the sense of being able to do something or knowing how to do something in the past. Survey the example sentences below.

  • Andy could run 100 kilometres in under three minutes when he was younger.
  • I could speak Spanish a little when I was in spain.
  • He could swim across this river when he was young.
  • She could recite John 3:16 when she was four.
Important Hint!  

Note that could only expresses ability to do something, but does not say that it was done. Hence, we should use was/were able to to talk about ability + action in the past.

Do not use could for a past action which really happened. i.e., “When the boat upset, they were able to swim to the bank.” (Not ... they could swim to the bank.)

2.  Expression of Habit

Could is also used to express past habit or character, as follows:

  • Father could be very strict at times.
  • I could leave nothing to chance.

3.  Expression of Polite Request

Using could to express polite request may take the following tones:

  • Could I have a glass of water?
  • Could you tell me how to solve this sum.
  • Could I use your telephone?
  • Could you go with your brother to the hospital?

4.  Expression of Permission

Could may also be used to express present, past or future permission usually in the following ways:

  • Could I bury my dog in your back yard?
  • Gretchen could not drive to the office today because her driving license had expired.
  • Could I smoke in here?
  • Do you think we could close the office early?
  • The principal said that the students could go home after the examination.

5.  Expression of Possibility

Could is also used to express possibility or ability in theoretical or factual circumstances, as shown below:

  • You could walk miles in the countryside without seeing a car.
  • I was so exhausted, I could sleep for a week. (now)
  • I was so exhausted, I could have slept for a week. (past)
  • The doorbell is ringing. It could be Mr. Wallace.
  • The road could be blocked.
Important Hint!  

Note that Could may be used with a present meaning when there is an idea of condition or speculation. Survey the following sentences.

  • Could you run the business single handed (if it was necessary)?
  • Could he get another job (if he left this one)?
  • Could you get there and back in one day (if you tried)?

Can / Could Comparison

Could is more unsure than can. Consider the following sentence.

  • There could be another rise in the price of gold.
    → (It is possible there will be)

Could is less sure than can. We must use could (not can) when we don’t really mean what we say:

  • I’m so angry with him. I could kill him. (not ‘I can kill him.’)

Can / Could Have

We use can/could have + participle to guess or speculate about what has happened, whether things (have) happened, etc.

Can is only used in questions and negative sentences Opens in new window, or with limiting words like only, hardly, or never. In other situations we use could. Survey the examples below.

  • Where can he have gone?
  • He can’t have gone to factory on a day of load shedding.

Could have + past participle is also used to talk about something that was possible, but did not happen. Observe the sentence below:

  • That was a bad place to go skating. You could have broken your leg.
Important Hint!  

In expressing ability, can and could frequently also imply willingness:

  • Can you help me with my homework?