Question

什么样的动词变为被动语态要后置to不定式？

Answer 1

有些动词主动形式后接不带to 的动词不定式，但是被动时则需要恢复to,比如常用的词make sb do sth ,被动be made to do sth。

关于不带to的不定式，举个例子：

They did not observe she come in and go upstairs。

被动：

She wasn't observed (by them) to come in and go upstairs。

带doing的就直接变换，因为doing的语法功能大体上相当于一个名词，及物动词直接加doing，不及物动词加介词再加名词/doing。

比如：

She was considered having lunch already.

She was devoted to studying computer.(这种情况很别扭,因为一般to doing的都是devote contribute,look forward,prefer之类的很主观的动词)

还有一个超经典的：be used to doing

She was used to sleeping at 9 o'clock.她已经习惯于9点睡觉.

这里边可不是被动语态，used大体相当个形容词