Usage of Can and Could
1. Indicating Ability
(1) To express current ability, use can:
My sister can drive. My sister can drive.
Everyone here can speak English. Everyone here can speak English.
(2) To express future ability, usually use the future tense of be able to instead of can or could:
I’ll be able to speak French in another few months. I will be able to speak French in a few months.
One day people will be able to go to the moon on holiday. One day people will be able to go to the moon for vacation.
However, if indicating a present decision about future ability, can can be used:
Can you come to the party tomorrow? Can you come to the party tomorrow?
(3) To indicate past ability, could can sometimes be used, but not always; pay attention to the following points:
① If indicating general past ability (i.e., the ability to do something at any time), could can be used:
Could you speak English then? Could you speak English at that time?
② If indicating specific past ability (i.e., the ability to do something in a specific situation in the past), could cannot be used; instead use was (were) able to do something, managed to do something, or succeeded in doing something:
He studied hard and was able to pass the exam. He studied hard and was able to pass the exam.
At last he succeeded in solving the problem. He finally solved the problem.
【Note】Could is not used to indicate specific past ability usually limited to affirmative sentences; in negative or interrogative sentences, it can indicate specific past ability:
I managed to find the street, but I couldn’t find her house. I managed to find the street, but I couldn’t find her house. (The first sentence uses managed to in an affirmative form, not could; the second sentence is negative, so could can be used)
Additionally, could can also be used with verbs of perception (such as see, hear, smell, taste, feel, understand, etc.) to indicate specific ability:
Looking down from the plane, we could see lights on the runway. Looking down from the plane, we could see lights on the runway.
In indirect speech, could can also indicate specific past ability:
He said he could see me next week. He said he could see me next week.
2. Indicating Permission
(1) For current or future permission, distinguish between the following two situations:
① To request permission (i.e., asking someone to allow oneself to do something), both can be used, but using could is more polite:
Can [Could] I come in? Can [Could] I come in?
【Note】At this time may or might can also be used, where might and could do not indicate the past, only a polite tone.
② To grant permission (i.e., allowing someone else to do something), generally only can is used, not could:
“Could [Can] I use your pen?” “Yes, of course you can.” “Can I borrow your pen?” “Of course you can.” (Cannot say Yes, you could.)
(2) For past permission, the following two situations should also be distinguished:
① To indicate general past permission (i.e., indicating someone could do something at any time), use the past form of can, which is could:
When I lived at home, I could watch TV whenever I wanted to. When I lived at home, I could watch movies whenever I wanted to. (General permission)
② To indicate specific past permission (i.e., indicating permission for a specific activity in a specific situation in the past), could cannot be used, and other expressions must be used (such as had permission, was [were] allowed to):
I was allowed to see the film yesterday evening. I was allowed to see the film yesterday evening. (Specific permission, could cannot be used)
3. Indicating Speculation
(1) For speculation about the present or future, both can be used, but can is usually only used in negative sentences or questions, generally not in affirmative sentences; while could can be used in affirmative, negative, and interrogative sentences:
It can’t be true. It can’t be true.
What can they be doing? What can they be doing?
We could go there this summer. We could go there this summer. (Future possibility)
You could be right, but I don’t think you are. You could be right, but I don’t think you are. (Present possibility)
【Note】Can is sometimes also used in affirmative sentences to indicate speculation, mainly seen in:
① Indicating theoretical possibility (i.e., theoretically possible, but may not actually happen):
Even experienced teachers can make mistakes. Even experienced teachers can make mistakes.
② Used in affirmative sentences containing only and other limiting expressions:
“Who is that at the door?” “It can only be the postman.” “Who is that at the door?” “It can only be the postman.”
③ Followed by “be (get, seem, become)+ adjective”, indicating “sometimes will”, “often will” etc:
It can get very hot here. It can get very hot here.
She can be very unpleasant. She can be very unpleasant.
(2) For speculation about the past, must follow can, could with the perfect form of the verb. However, can is still only used in negative or interrogative sentences, not in affirmative sentences; while could can be used in various sentence types:
He can’t [couldn’t] have seen her there. He couldn’t have seen her there.
He could have gone home. He could have gone home.
Who could have taken it? Who could have taken it?
I do not see how I could have done otherwise. I do not see how I could have done otherwise.
She could not have been more than sixteen. She could not have been more than sixteen.
I couldn’t have won, so I didn’t go in for the race. I couldn’t have won, so I didn’t participate in the race.
【Note】Could followed by the perfect form of the verb, in addition to indicating speculation about the past, has the following uses (not using can):
① Indicating a possibility that did not happen in the past (i.e., something that could have happened but did not):
He could have told her, but he didn’t choose to. He could have told her, but he didn’t choose to.
I could have lent you the money. Why didn’t you ask me? I could have lent you the money. Why didn’t you ask me?
② Used to politely blame someone for not doing something they should have done in the past:
You could have been a little more considerate. You could have been a little more considerate.
You could ask me before you use my phone. You could ask me before you use my phone.
You could have helped me why did you just sit and watch? You could have helped me; why did you just sit and watch?
③ Indicating “almost about to”:
I could have died laughing. I could have died laughing.
I was so angry I could have killed him. I was so angry I could have killed him.
She felt miserable. She could have cried. She felt miserable. She could have cried.
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