剑桥KET语法：情态动词表示应该

　　情态动词可以表示应该，意为“必须”“应当”等，常见的相关情态动词有must 、should 、need 等。

　　1、must

　　must 表示“必须” “应该”,否定式是must not,缩写为 mustn't, 意为“不应该” “禁止”,如：

　　You mustn't play with fire.

　　你不应该玩火。

　　2、should

　　should 表示建议或劝告，意为“应该”,如：

　　You should go to see the doctor.

　　你应该去看看医生。

　　3、need

　　need作情态动词时，常用于疑问句、否定句和条件句中，此外，回答用need提问的句子时，肯定回答用must, 否定回答用needn't,如：

　　—Need I help you? 需要我帮助你吗?

　　一Yes,you must./No,you needn't.是的，你必须要。/不，你没必要。

　　4、have to

　　have to 表示“不得不”,否定式是 don't have to, 意为“不是必须的”,如：

　　I can't find my umbrella,so I have to buy a new one.

　　我找不到我的雨伞了，所以我不得不买一把新伞。

　　5、单项选择：请从下列选项中选出正确的选项。

　　1.—Must I finish my homework before seven o'clock?

　　—Yes,you ___________

　　A.may B.can C.must

　　2.—Must we clean the house now?

　　—No,you ___________

　　A.needn't B.mustn't C.can't

　　3.—Need I return the book now?

　　—No,you ___________ .You can keep it till next week if you like.

　　A.can't B.mustnt C.needn't

　　4.Tom,you ___________ play with the knife.You ___________ hurt yourself.

　　A.needn't;can B.mustn't;may C.can't;must

　　5.—Mum,I have a toothache.

　　You ___________ see a dentist

　　A.should B.would C.might

　　6. ___________ our children be taught to swim at school?

　　A.Should B.Would C.Might

　　6、请从括号内选择正确的单词填空。

　　1.You(can/must) ___________ go and see a doctor because you've got a fever.

　　2.—Shall I get one more cake for you?

　　—Thanks,but you(can't/needn't) ___________ ,I've had enough.

　　3.The children (can't/mustn't) ___________ play football on the road.

　　4.—Must I finish my homework now?

　　—No,you(needn't/mustn't) ___________

　　5.My mother spent the whole afternoon in cleaning the house.She(must/can) ___________ be tired.

　　6.You(could/should) ___________ have no problem with reading this language.

　　7.You(shouldn't/needn't) ___________ bother to return the book since we'll meet tomorrow.

　　8.Buying budget-priced furniture(mustn't/needn't) ___________ mean compromising on quality or style.