shall,是用于第一人称的,即I和we,在建议第二人称的人（you)时,should.而且should,也是shall的过去式
例：你应该去看看医生—You should go to see a doctor.
我们应该坐下—we should sit down.
通常用should比用shall多,而且should的语气比较委婉.
must与 be 连用,表示猜测、推测,在使用中要注意这个情态动词短语的两个方面.
1、反意疑问句：主语 must be..., be not 主语（代词）?类似的用法还有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 与may be 的否定形式为 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 与行为动词连用,在变为反意疑问句的时候,就可以直接使用mustn't .
eg. He must finish it at once, mustn't he?