本单元语法内容主要是学习had better, should等情态动词，现将它们的用法讲解如下：
1. had better
had better意为“最好”，可简写为’d better，其后接动词原形。可用于提出建议、劝告或命令等，含有“应该做”之意，不十分客气，故一般只用于对晚辈或平辈，不用于对长辈。例如：
I had better go and see a doctor.
You’d better have another try before you give it up.
有时，为缓和语气，使其更客气，可加I think。例如：
I think you’d better not cut down any of those trees.
其否定式是had better not do sth.，其疑问式使用had构成。例如：
You’d better not smoke here.
Had she better lie in bed and have a good rest?
美国口语中常用better代替had better。例如：
Better stay indoors today.
2. should
should意为“应该”，可用来表示劝告、建议等。例如：
You should listen to your parents’ words.
You should read the passage carefully.
should还可用来表示明显的结果或合理的推论，意为“应该;理应”。例如：
Mary took dancing lessons for years; she should be an excellent dancer.
在疑问句中，should可用来表示令人惊讶、不合理或难以置信的事情。例如：
How should I know?
Why should you be sorry about the news?
有时，should可表示郑重、客气的意思，但多用于第一人称。例如：
I should say that the man is over 60.
另外，should还可用于虚拟语气。例如：
They suggested that we should have an early start.
should like to do sth.同would like to do sth.，意为“愿意做某事”。例如：
I should like to go on a diet.
“should＋have＋过去分词”结构可表示对过去所发生的事的看法, should have done sth.表示本应做某事而没有做，shouldn’t have done sth.表示做了本不应做的事。例如：
You should have told me about it. 你本应告诉我这件事。(实际没告诉)
You shouldn’t have gone there yourself. 你本不应该自己去那里。(自己去了)
3. ought to
ought to意为“应当;应该”，其含义相当于should，但语气更强，且多与责任、义务相关。例如：
You ought to help each other in your study.
Students ought to hand in their homework on time every day.
其否定式为ought not to do sth.，疑问式使用ought构成。例如：
You oughtn’t to be so rude to your parents.
Ought I to go with you?
ought to可用于表示推测，意为“应该会；按理应”。例如：
Today is Monday. Mr Black ought to be at school.
ought to have done sth.同should have done sth.，表示该做的事而未做, oughtn’t to have done sth.同shouldn’t have done sth.，表示做了不该做的事。例如：
You ought to have told me about it earlier. 你本应早点告诉我此事。(未告诉)
You oughtn’t to have gone there yourself. 你本不应该自己去那里。(自己去了)
