need 作實義動詞
On the one hand, it can be used as an ordinary verb and has the same forms as an ordinary verb. The third person singular ends with an s, and questions and negatives are formed with do. As an ordinary verb, need is normally followed by an infinitive with to:
一方面，need可用作實義動詞，此時它的用法跟其他動詞一樣：第三人稱單數作主語時加s，疑問句和否定句用do來構成。做實義動詞時，need一般接帶to的不定式。例句：
A: He needs to see a doctor. Do you need to see one too?
A: 他要去看醫生。你要去嗎？
B: No, I don't need to see a doctor. But I need a good rest.
B: 不，我不需要看醫生。但我要好好休息一下。
In this sense, when we are talking about necessity, we generally prefer need to / do not need to.
從這個角度講，當我們談論「必須」時，我們一般傾向於使用need to / do not need to（也就是將need作為實義動詞用）。
need 作情態動詞
In the other format, need behaves as a modal verb, like can, could, must, should, etc. It has the same forms as modal auxiliary verbs: the third person singular has no s, questions and negatives are made without do. In this format, need is followed by an infinitive without to:
另一方面，need還可用作情態動詞，就像can, could, must, should等詞那樣。此時它的用法同其他情態助動詞一樣：第三人稱單數作主語時不加s，疑問句和否定句不用do來構成。做情態動詞時，need一般接不帶to的不定式。例句：
A: Need I lock the door when I leave?
A: 我走時要鎖門嗎？
B: No, you needn't. Sarah will be home soon.
B: 不，不用。Sarah馬上就回來了。
In this sense, we are talking more about obligation and giving permission to someone not to do something. Note also that as a modal verb it is most commonly used in negative sentences and sometimes in questions.
用作情態動詞時，我們所談論的更多是指義務責任以及同意某人不做某件事情。同時要注意，作情態動詞時need通常用於否定句，有時用於疑問句。（也就是說不用於肯定句）
比較
Compare the following alternatives to needn't and don't need to in the examples below:
試比較下面對於例句中 needn't 和 don't need to 的替換用法：
1. You needn't come if you don't want to.
如果你不想來就不必來。
Alternative: Don't feel obliged to come if you don't want to.
換言之：如果你不想來，不用覺得你有義務要來。
2. You don't need to be a genius to see that it is wrong.
你不必是個天才也能看出它是錯的。
Alternative: You don't have to be a genius to see that it is wrong.
換言之：並不是說你必須是個天才才能看出它是錯的。
實習生：李帥男