should，must / mustn't 的用法

should，must / mustn't 的用法

（1）should表示建议、责任、义务、合适性或可能性等，也表示推测、可能。

The city should clean the streets more often.

They should be here by 8:00.

（2）must侧重说话人的主观看法或意见，表示义务、要求或命令。即“说话人认为某人是必须的、必要的”，常翻译为“必须”。其否定形式为mustn't，意思是“不许，不要，不准”，表示禁止之意。回答must提出的问题时，肯定用must；否定用needn't或don't have to。

We must remember our past and build our future.

Children mustn't play with fire.

------ Must I come tomorrow?

------ Yes, you must.

------ No, you needn't / don't have to.

注意：must还可以用来表示推测，其肯定程度比may，might，could大得多，一般只用于肯定句中。否定、疑问句中要用can，could来代替must。如：

表示对现在的情况的推测：must ＋ do

表示对正在进行的动作或状态的推测：must＋be doing

表示对过去的事情的推测：must ＋ have done

You must be feeling cold with only a shirt on.

The ground is wet. It must have rained last night.

He's just had his lunch. He can't be hungry.

She couldn't have been more than 18 years old at that time.