1. can
表示能力、许可、推测或可能性。例如：
The baby can't dress himself.（表示能力）
Can I watch TV tonight? 我今晚可以看电视吗？（表示许可，相当于may）
He can't be at home. 他不可能在家。（表示推测）
Can the news be true? 这消息可能是真的吗?（表示可能性）
2. must
表示“必须”作某件事情, 没有其他选择；否定式mustn’t表示“禁止”、“不准”。
Henry is going on an airplane. He must have a ticket. 亨利要上飞机了, 他非要有飞机票不可。
Peter mustn't eat lunch in class.
Must I pay now? 我必须现在就付款吗？(如回答不必时，要用needn't或don't have to)
3. should
表示“应该”，用来劝说别人应该做某事。例如：
It's cold. You should wear a sweater. 天冷了, 你应该穿毛衣。
You shouldn't judge a man always by the clothes. 你不应该总是以貌取人。
下面请同学们做一个练习：阅读下面的短文，找出情态动词并在下面画线，并说明其用法。
Every week Peter and Linda can receive five dollars from their parents. It is not a gift. Peter and Linda must work for the family to earn the money. Peter takes out the garbage and cleans the yard around the house. He should do it everyday, but sometimes he forgets. Linda helps her mother with the cooking. She also washes the dishes after dinner. Peter and Linda mustn’t(don't have to) use their money for food. They use it for their entertainment. Sometimes they go to the movies. Sometimes they buy records. Each week they save some of their money to buy something more expensive.
