need 作实义动词
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可用作实义动词，此时它的用法跟其他动词一样：第三人称单数作主语时加s，疑问句和否定句用do来构成。做实义动词时，need一般接带to的不定式。例句：
A: He needs to see a doctor. Do you need to see one too?
A: 他要去看医生。你要去吗？
B: No, I don't need to see a doctor. But I need a good rest.
B: 不，我不需要看医生。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.
从这个角度讲，当我们谈论“必须”时，我们一般倾向于使用need to / do not need to（也就是将need作为实义动词用）。
need 作情态动词
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还可用作情态动词，就像can, could, must, should等词那样。此时它的用法同其他情态助动词一样：第三人称单数作主语时不加s，疑问句和否定句不用do来构成。做情态动词时，need一般接不带to的不定式。例句：
A: Need I lock the door when I leave?
A: 我走时要锁门吗？
B: No, you needn't. Sarah will be home soon.
B: 不，不用。Sarah马上就回来了。
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.
用作情态动词时，我们所谈论的更多是指义务责任以及同意某人不做某件事情。同时要注意，作情态动词时need通常用于否定句，有时用于疑问句。（也就是说不用于肯定句）
比较
Compare the following alternatives to needn't and don't need to in the examples below:
试比较下面对于例句中 needn't 和 don't need to 的替换用法：
1. You needn't come if you don't want to.
如果你不想来就不必来。
Alternative: Don't feel obliged to come if you don't want to.
换言之：如果你不想来，不用觉得你有义务要来。
2. You don't need to be a genius to see that it is wrong.
你不必是个天才也能看出它是错的。
Alternative: You don't have to be a genius to see that it is wrong.
换言之：并不是说你必须是个天才才能看出它是错的。