could (kŏŏd; unstressed kəd), v. a pt. of can 1. auxiliary verb. (used to express possibility): I wonder who that could be at the door. That couldn't be true. (used to express conditional possibility or ability): You could do it if you tried. (used in making polite requests): Could you open the door for me, please?
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.
"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."
When you’re talking about the possibility of something happening in the future, use can if something is very likely to happen. Could, on the other hand, should be used for things that have an equal chance of not happening.