情态助动词有一定意义，但意义不完整，因此不能独立作谓语，需与实义动词一起构成谓语。情态助动词总共有9个：can, may, must, will, shall, need, dare, ought to, used to。



推测性(表示可能性大小)
It is not Tom. (0%)否定判断          It might be Tom. (5%)
It may be Tom. (10—20%)         It could be Tom. (30—40%)
It can be Tom. (50%)                   It should be Tom. (60%)
It ought to be Tom. (70%)           It would be Tom. (80%)
It will be Tom. (90%)                   It must be Tom. (95%)
It is Tom. (100%)肯定判断


非推测性(不表示可能性)
He said that I might go then.(过去可以)
You may go now.(现在可以)
He could speak English when he was young.(过去的能力)
He can speak English.(现在的能力)
You should speak English in class.(应该，语气弱)
You ought to speak English in class.(应该，语气强)
He said that he would go to Beijing the next day.(过去将来)
I will go to Beijing tomorrow.(一般将来)
You must speak English in class.(必须)