Period three Grammar

should 和ought to

1. should和ought to表示责任、建议或劝告，翻译为"应该"，后者语气强一些，可用于各种人称。

You should listen to the doctor's advice if you want to recover soon.

Such things ought not to be allowed.

2. should接动词的完成式，是一种谴责，表示过去应该做而没有做或过去做了而不应该做。　　

I missed the class. I should have come earlier.

I shouldn't have made such a foolish mistake.

3. 表示要求，命令时，语气由 should（应该）、had better最好）、must（必须）渐强。

have to和must

1. 两词都是'必须'的意思，have to 表示客观的需要， must 表示说话人主观上的看法，既主观上的必要。

My brother was very ill, so I had to call the doctor in the middle of the night.　(客观上需要做这件事)

He said that they must work hard.（主观上要做这件事）

2. have to有人称、数、时态的变化，而must只有一种形式。但must 可用于间接引语中表示过去的必要或义务。

He had to look after his sister yesterday.

3. 在否定结构中： don't have to　 表示"不必"; mustn't　表示"禁止"

You don't have to tell him about it.

You mustn't tell him about it.　　　

need和have to

1. need表示需要，主要用于否定句和疑问句。用于疑问句，肯定回答用must，否定回答用needn't have to表示他人或客观情况要求"必须"，也可以翻译为"不得不"，have to否定形式意思是"不一定"或"没有必要"。

You needn't come if you don't want to.