书上说,WHAT引导的主语从句,谓语一般用单数形式,如果谓语动词后的表语是复数形式时,谓语用复数形式.

一、用 what 引导的主语从句究竟表示单数还是复数,需要从代表的概念上进行判断,其判断方法就是看它的表语是单数还是复数,如：
What she told me is none of your business. 他所告诉我的与你无关.（none of your business 属于单数的抽象概念,因此用动词 is 与主语保持一致）
What I need for the spring cleaning are a pair of rubber gloves and some plastic bags . 大扫除我所需的是一双橡皮手套和几个塑料袋. （既然既有手套又有塑料袋,说明所需的不是单数而是复数,因此动词用 are 与其保持一致.）
What she wanted from him were just promises. 她想从他得到的只是一些承诺.（希望的得到的是若干个承诺,因此 用 were 与主语保持一致）
二、其实以上的句子倒过来改写如下就会一目了然：
None of your business is what she told me. 与你无关的是她所告诉我的.
A pair of rubber gloves and some plastic bags are what I need for the spring cleaning. 一双塑料手套和若干塑料袋是我大扫除所需的.
Some promises were just what she wanted from him . 一些承诺仅只是她想从他得到的.

“what”引导主语从句时，谓语动词分为两种情况：
1）常与其后的名词作表语一致
2）根据句子的语境而定
例如：
What you left are only several old books.
What you said is of great importance.