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?