句子结构的问题

Question

Professor Ronald Bracewell, a leading American radio astronomer, argued in Nature that such a superior civilization, on a visit to our own solar system, may have left an automatic messenger behind to await the possible awakening of an advanced civilization. Such a messenger, receiving our radio and television signals, might well re-transmit them back to its home-planet, although what 【impression any other civilization would thus get from us 〔is best left unsaid〕. 】

问题：句子what impression any other civilization would thus get from us is best left unsaid. 可以简化成 the impression is best left unsaid.

简化后的句子能不能转化为这样？：It is best that we leave the impression unsaid.

如果可以转化，替换掉形式主语it后，句子变成：we leave the impression unsaid is best. we leave the impression unsaid是主语从句。

再把主语从句转化为被动语态后，句子变成：the impression is left unsaid is best.

我自己分析到这一步后，就想不出怎样才能把句子还原为原来的结构了，请您指点一下，感谢！

Answer 1

问题出在第二步。the impression is best left unsaid是被动语态，其中best是状语。还原的句子应该是we best leave the impression unsaid (当然这不是正常的表达)。你的第二步是另造了一个句子。从改过的句子看，unsaid是宾语补语。

原句中what impression any other civilization would thus get from us是what引导的主语从句，谓语是is left，最后的unsaid是主语补语。

不知你要怎么”还原”这句话？来自：求助得到的回答

Answer 2

替换形式主语后的句子是错误的，如果把形式主语中的it替换掉的话，就应该变成：That we leave the impression unsaid is best. 这一步中的that不可以省略掉。此时，主语从句由that引导。变成被动以后就是That the impression is left unsaid is best.

that之所以不可以省略是因为这里的主语必须是一个从句，that就是起引导这个从句的功能。如果去掉that变成we leave the impression unsaid is best. we leave the impression unsaid 本身就是一个主谓完整的句子，而后面又接了一个is unsaid，再次出现谓语(is),这就变成句子里的句子，说不通了。