From Longman Dictionary of Contemporary English could /kəd; strong kʊd/ S1 W1 modal verb (negative short form couldn’t) 1 past ability used as the past tense of ‘can’ to say what someone was able to do or was allowed to do in the past By the time she was eight, she could read Greek and Latin.
The meaning of COULD is —used in auxiliary function in the past, in the past conditional, and as an alternative to can suggesting less force or certainty or as a polite form in the present. How to use could in a sentence.
We use could to ask for permission. Could is more formal and polite than can: Could I ask you a personal question?
We use could to show that something is possible, but not certain: They could come by car. (= Maybe they will come by car.) They could be at home. (= Maybe they are at home.) We use can to make general statements about what is possible: It can be very cold here in winter. (= It is sometimes very cold here in winter.)
The negative form of could is could not or couldn't. To form the negative of be able to, you either put not or another negative word in front of able, or you use the expression be unable to.
Definition of could modal verb in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more.
"Could" is a modal verb used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can."