英語中有三個不定代詞，它們只用于兩者，不用于表示三者。這三個不定代詞是 both, either, neither。這三個不定代詞在中考英語考題中經常被考查到，且很容易出錯，所以同學們應引起高度注意。

both, either, neither 這三個不定代詞都用來談論兩者，其中both 意為“(兩者)都”， either意為“(兩者中)任意一個”，neither 意為“(兩者)都不”。若要指三者或三者以上，分別用 all 意為“(三者)都”，any 意為“(三者中)任何一個”，none 意為“(三者)都不”等。比較：

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.

這三個表示“兩者”的不定代詞除用作代詞，還可用作形容詞。注意，用作形容詞時，both 后接復數名詞，而 either 和 neither 之后要接單數可數名詞。如：

Both of the brothers are here. / Both brothers are here. 兄弟倆都在這兒。

Either of the books will do. / Either book will do. 兩本書中哪一本都行。

Neither of the stories is interesting. / Neither story is interesting. 兩個故事都沒有趣。

注：不定代詞 both, either, neither 用作主語時，both 之后的謂語動詞總是復數，either 和 neither 后的謂語動詞通常用單數。