1.在肯定句中一般用must/may/might/could。
He must/may/might know the answer to this question.
他一定/可能/也許知道這個問題的答案。
It is cold in the room.They must have turned off the heating.
屋里很冷,他們肯定把暖氣關了。
2.否定句中用can’t/couldn’t,may not/might not。
It can’t/couldn’t be the headmaster.He has gone to America.
這不可能是校長,他去美國了。
He may not/might not know the scientist.他也許不認識那位科學家。
3.疑問句中用can/could。
Could he have finished the task? 他可能把任務完成了嗎?
Can he be at home now? 他現(xiàn)在能在家嗎?