这个例句是解释比较分句省略主语而保留谓语动词的制约因素。原文及例句如下:
Restrictions on omission of subject
In clauses where the verb is retained, the subject can be omitted only when it is the counterpart to the comparative phrase or is understood as an embedded clause, as in (d) above:
[22] i More people came than [ were invited ]. [counterpart to comparative phrase]
ii He spent longer on it than [ seemed necessary]. [embedded clause]
iii ∗ Liz works harder than [ worked/did last year].
To remedy [iii] we must either insert a subject (she) or else omit the verb too, giving than last year.
作者清楚告诉读者,than分句省略主语而保留谓语动词只能是二种情况之一:1 省略的主语为比较对应项;2 省略的主语为嵌入分句。除此以外省略主语而保留谓语动词即为语法错误。
在这个句子中 He spent longer on it than seemed necessary. 作者已经告诉我们seemed省略的主语为嵌入分句,即than (it) seemed necessary (that he should spent on it/for him to spend on it). 补上省略的主语就是补上省略的嵌入分句(形式主语it+外置的that主语从句或不定式分句)。作者并不认为这里省略的是what。所以如果加上what,会被作者认为是错误的。