有一位读者来信说：有这样两句话：
(1)She is sorry for what she had said.
(2)With these words I must leave you, for how long I know not.
请问如何分析 for what she had said 和 for how long I know not?
首先让我们探讨一下上述句（1）中的 for what she had said。这是一个介词短语，由介词 for 加上一名词性从句构成。“介词+名词性从句”结构在英语里并不罕见。在这种结构中，介词常可后接由缩合连接代词 what 和缩合连接副词 when, where 等所引导的名词性从句。上述句（1）中的 for what she had said 即是“介词后接由缩合连接代词 what引导的名词性从句”结构的一例，再如：
(3)He is aware of what I write .
“介词后接由缩合连接副词 when 引导的名词性从句”结构的例证有如：
(4)She wore the same black shiny raincoat as when they'd first met .
“介词后接由疑问词引导的名词性从句”（即间接疑问句），如：
(5)This article will give you some idea of what relativity means .
(6)No rule can be given as to when it will happen .
(7)We have no definite information yet as to which route he will take .
(8)The boys show great interest about how the machine works .
介词亦可后接由 whether 引导的名词性从句，如：
(9)There is a continuing controversy over whether Utopian socialism has ever existed in modern China .
介词还可后接由感叹词 how 引导的间接感叹句，如：
(10)She was surprised at how pretty the little house was .
现在让我们再分析一下上述句（2）中的 for how long I know not。这是一个与前一分句（即 with these words I must leave you）并列的分句。此一分句的基本结构是“主语+谓语+宾语”：主语为人称代词 I； 谓语为 know not （相当于 do not know, 仅较为文气而已）；宾语为 for how long。
for how long 是一省略结构，等于 for how long I must leave you。 这显然是由 for how long 引导的一间接疑问句，用作宾语，由于与上文关系密切而置于分句之首。for how long 本身自然是一介词短语，用作时间状语。所以在这里介词 for 的宾语是 how long,而不是一个名词性从句，这正是与上述所讨论的“介词+名词性从句”结构的不同之处。
与上述 for how long I know not 相似的结构也常用作各种状语置于单一的分句或句子之中，如：
(11)It was as though her mother had been right and a vengeful God was punishing Jill for she knew not what .（用作原因状语）
(12)A crowd instantly gathered from nobody knew where .（用作地点状语）
最后，不妨顺便提一下，在某些名词和形容词之后，上述“介词+名词性从句”结构中的介词往往可以省去，如：
(13)You must take care how you drive your car along busy streets. （名词 care后省去复合介词 as to）
(14)I was curious why the motor was off. （形容词 curious 后省去介词 about）
