英语中有三个不定代词，它们只用于两者，不用于表示三者。这三个不定代词是 both, either, neither。这三个不定代词在中考英语考题中经常被考查到，且很容易出错，所以同学们应引起高度注意。

both, either, neither 这三个不定代词都用来谈论两者，其中both 意为“(两者)都”， either意为“(两者中)任意一个”，neither 意为“(两者)都不”。若要指三者或三者以上，分别用 all 意为“(三者)都”，any 意为“(三者中)任何一个”，none 意为“(三者)都不”等。比较：

He has two sons; both of them are clever.

He has three sons; all of them are clever.

He has two sons; either of them is clever.

He has three sons; any of them is clever.

He has two sons; neither of them is clever.

He has three sons; none of them are clever.

这三个表示“两者”的不定代词除用作代词，还可用作形容词。注意，用作形容词时，both 后接复数名词，而 either 和 neither 之后要接单数可数名词。如：

Both of the brothers are here. / Both brothers are here. 兄弟俩都在这儿。

Either of the books will do. / Either book will do. 两本书中哪一本都行。

Neither of the stories is interesting. / Neither story is interesting. 两个故事都没有趣。

注：不定代词 both, either, neither 用作主语时，both 之后的谓语动词总是复数，either 和 neither 后的谓语动词通常用单数。