用于表示“需要，必要”之意。做情态动词时，仅用于否定句和疑问句，只有现在时，过去式要用needn't have，疑问式用need+人称?,否定式用need not(即needn't），
1．—Need we leave soon?
—Yes, you must.(No, you needn't)
2．You needn't have hurried.
(=It was not necessary for you to hurry，but you did).你当时不必这么匆忙。
3.He needs to see a doctor.(need to do)
在肯定句中need 不用作情态动词
You needn't go to school, for today is Sunday.
Need I finish the work today?
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