could

Từ điển Oxford Advanced Learner 8th

could [could] BrE [kəd] NAmE [kəd] BrE strong form [kʊd] AmE strong form [kʊd] modal verb (negative could not, short form couldn'tBrE [ˈkʊdnt] ; NAmE [ˈkʊdnt] )
1. used as the past tense of ‘can’
• She said that she couldn't come.
• I couldn't hear what they were saying.
•Sorry, I couldn't get any more.
2. used to ask if you can do sth
• Could I use your phone, please?
• Could we stop by next week?
3. used to politely ask sb to do sth for you
• Could you babysit for us on Friday?
4. used to show that sth is or might be possible
• I could do it now, if you like.
• Don't worry— they could have just forgotten to call.
•You couldn't have left it on the bus, could you?
•‘Have some more cake.’ ‘Oh, I couldn't, thank you (= I'm too full).’
5. used to suggest sth
• We could write a letter to the director.
• You could always try his home number.
6. used to show that you are annoyed that sb did not do sth
• They could have let me know they were going to be late!
7. (informal)used to emphasize how strongly you want to express your feelings
• I'm so fed up I could scream!
Idiom: ↑could do with something

Grammar Point:
modal verbs
The modal verbs are can, could, may, might, must, ought to, shall, should, will and would. Dare, need, have to and used to also share some of the features of modal verbs.
Modal verbs have only one form. They have no -ing or -ed forms and do not add -s to the 3rd person singular form: ▪ He can speak three languages. ◇ ▪ She will try and visit tomorrow.
Modal verbs are followed by the infinitive of another verb without to. The exceptions are ought to and used to: ▪ You must find a job. ◇ ▪ You ought to stop smoking. ◇ ▪ I used to smoke but I gave up two years ago.
Questions are formed without do/does in the present, or did in the past: ▪ Can I invite Mary? ◇ ▪ Should I have invited Mary?
Negative sentences are formed with not or the short form -n’t and do not use do/does or did.
You will find more help with how to use modal verbs at the dictionary entries for each verb.

Grammar Point:
can / could / be able to / manage
Can is used to say that somebody knows how to do something: ▪ Can you play the piano? It is also used with verbs of seeing, noticing, etc: ▪ I can hear someone calling, and with passive infinitives: ▪ The DVD can be rented from your local store.
Can or be able to are used to say that something is possible or that somebody has the opportunity to do something: ▪ Can you/are you able to come on Saturday?
You use be able to to form the future and perfect tenses and the infinitive: ▪ You’ll be able to get a taxi outside the station. ◇ ▪ I haven’t been able to get much work done today. ◇ ▪ She’d love to be able to play the piano.
Could is used to talk about what someone was generally able to do in the past: ▪ Our daughter could walk when she was nine months old.
You use was/were able to or manage (but not could) when you are saying that something was possible on a particular occasion in the past: ▪ I was able to/managed to find some useful books in the library. ◇ I could find some useful books in the library. In negative sentences, could not can also be used: ▪ We weren’t able to/didn’t manage to/couldn’t get there in time. Could is also used with this meaning with verbs of seeing, noticing, understanding, etc: ▪ I could see there was something wrong.
Could have is used when you are saying that it was possible for somebody to do something in the past but they did not try: ▪ I could have won the game but decided to let her win.