2. 在否定句中用can’t/couldn’t (不可能)，may not/ might not(可能不)。 e.g. It can’t /couldn’t be the headmaster. He has gone to America. e.g. He may not/might not know the scientist. 3. 在疑问句中，一般只使用can/could。 e.g. Could he have finished the task? e.g. Can he be at home now?