shall,是用于第一人稱的,即I和we,在建議第二人稱的人（you)時(shí),should.而且should,也是shall的過去式
例：你應(yīng)該去看看醫(yī)生—You should go to see a doctor.
我們應(yīng)該坐下—we should sit down.
通常用should比用shall多,而且should的語氣比較委婉.
must與 be 連用,表示猜測、推測,在使用中要注意這個(gè)情態(tài)動(dòng)詞短語的兩個(gè)方面.
1、反意疑問句：主語 must be..., be not 主語（代詞）?類似的用法還有may be.
eg. They must be late, aren't they?
It may be in your pocket, isn't it?
2、否定句：must be 與may be 的否定形式為 can't be.
eg. They must be late. --(否定)They can't be late
It may be in your pocker. --(否定)It can't be in your pocket.
must 與行為動(dòng)詞連用,在變?yōu)榉匆庖蓡柧涞臅r(shí)候,就可以直接使用mustn't .
eg. He must finish it at once, mustn't he?