關(guān)于what引導(dǎo)的主語從句
先看這樣一個(gè)句子,In general,the tests work most effectively when the qualities to be measured can be most precisely defined and least effectively when what is to be measured or predicted cannot be well defined.
有些問題不確定需要得到證實(shí)吧.
(我自己分析的不知道是否正確),
the tsets work  most effectively .and least effectively.主句
 to be measured 做定語
what引導(dǎo)的主語從句,也就相當(dāng)于the qualities that.
那為什么不能把what直接換成和第一個(gè)when后面的the qualities呢?如果能換的話第二個(gè)when引導(dǎo)的狀語從句里面有2個(gè)動(dòng)詞,就矛盾了.
我的理解是否正確,這是考研試題的句子,有點(diǎn)小難.