1. Indicating Ability
(1) To indicate present ability, use can:
My sister can drive.
Everyone here can speak English.
(2) To indicate future ability, usually do not use can or could, but use the future tense of be able to:
I’ll be able to speak French in another few months.
One day people will be able to go to the moon. However, if indicating the ability to do something in the future based on a present decision, can can be used:
Can you come to the party tomorrow?
(3) To indicate past ability, sometimes could can be used, but sometimes it cannot. Specifically, pay attention to the following points:
① If indicating general past ability (i.e., the ability to do something whenever one wanted), could can be used:
Could you speak English then?
② If indicating specific past ability (i.e., the ability to do something in a specific situation in the past), could cannot be used, but was (were) able to do or managed to do or succeeded in doing can be used.
He studied hard and was able to pass the exam.
At last he succeeded in solving the problem.
Note: Could is typically not used to indicate specific past ability in affirmative sentences, but it can be used in negative or interrogative sentences:
I managed to find the street, but I couldn’t find her house.
Additionally, could can also be used with verbs of perception (e.g., see, hear, smell, taste, feel, understand, etc.) to indicate specific ability:
Looking down from the plane, we could see lights.
In indirect speech, could can also indicate specific past ability:
He said he could see me next week.
2. Indicating Permission
(1) For present 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 could is more polite:
Can [Could] I come in?
Note: May or might can also be used here, where might and could do not indicate the past but only express politeness.
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.”
(2) For past permission, distinguish between the following two situations:
① To indicate general past permission (i.e., indicating that someone could do something at any time), use the past tense of can, which is could:
When I lived at home, I could watch TV whenever I wanted to.
② To indicate specific past permission (i.e., indicating that a specific activity was allowed in a specific situation in the past), could cannot be used, and other expressions (like had permission, was [were] allowed to) should be used:
I was allowed to see the film yesterday evening.
3. Indicating Speculation
(1) For present or future speculation, both can be used, but can is usually only used in negative sentences or questions, and generally not in affirmative sentences, while could can be used in affirmative, negative, and interrogative sentences:
It can’t be true.
What can they be doing?
We could go there this summer.
You could be right, but I don’t think you are.
Note: Can is sometimes also used in affirmative sentences to indicate speculation, which mainly occurs in:
① Indicating theoretical possibility (i.e., it is theoretically possible, but may not actually happen):
Even experienced teachers can make mistakes.
② Used in affirmative sentences containing limiting words like only:
“Who is that at the door?” “It can.”
③ Followed by “be (get, seem, become) + adjective”, indicating that something occurs sometimes:
It can get very hot here.
She can be very unpleasant.
(2) For past speculation, it must follow can or 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 could have gone home.
Who could have taken it?
I do not see how I could have done otherwise.
She could not have been more than sixteen.
I couldn’t have won, so I didn’t go in for the race.
Note: Following could with the perfect form of the verb, besides indicating past speculation, has the following usages (not using can):
① Indicating a possibility that did not materialize in the past (i.e., something that could have happened but did not):
He could have told her, but he didn’t choose to.
I could have lent you the money. Why didn’t you ask me?
② Used to politely reproach someone for not doing something they should have done in the past:
You could have been a little more considerate.
You could ask me before you use my phone.
You could have helped me; why did you just sit and watch?
③ Indicating “almost”:
I could have died laughing.
I was so angry I could have killed him.
She felt miserable. She could have cried.
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