叫人家不必做一件事，除了说：It is not necessary (for you) to do that，你当然还可以用名词need字：There is no need (for you) to do that。用need做动词则怎样说呢？ Need做动词，用法比较特别。首先，它可以和普通动词一样有字尾变化，例如： You need. / He needs to do that.（你/他必须这样做）、You don’t . / He doesn’t need to do that. (你/他不必这样做)、Do you / Does he need to do that?（你/他要这样做吗？）
但在否定或疑问句里，need字有时会做辅助动词(auxiliary verb)用，和must、should等差不多，例如You / He need not do that. (=You don’t. / He doesn’t need to do that.)、Need you / he do that? (=Do you / Does he need to do that?) 这两句虽然是说现在的事，但need和he连用也不必加s。
说过去，You did not need to do that. 是说‘你不必那样做’，做了没有未见说明，need是主要动词；You need not have done that. 也是说‘你不必那样做’，但你做了，need是辅助动词。
还有一个字是必须谈谈的：necessarily。这是个副词(adverb)，和形容词(adjective) necessary 同源而形似，但意思不同，解作‘必然’，常用于否定句，not necessarily即‘未必’，例如：(1) “He’ll lose.” “Not necessarily.” ( ‘他会输。’ ‘未必。’) (2) Learned people do not necessarily prosper.（有学问未必就会发）。