1. Definition of Cannot (modal verb): negative form of can

2. Could "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."

3. The modal verb Could is most often used as a past-tense version of can, indicating what someone or something was able to do in the past; it can also be used instead of can as a more polite way of making a request or asking for permission

4. From Longman Dictionary of Contemporary English Could Could / kəd; strong kʊd / S1 W1 modal verb (negative short form Couldn’t) 1 past ability CAN 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