為什麼有的句子it+is有的只要it 例如 It embarrassed Bob to be mistaken foe a girl. 為什麼it後面沒有is?

It embarrassed Bob to be mistaken for a girl.
= To be mistaken for a girl embarrassed Bob.
兩句的主要動詞都是embarrassed
因為不定詞片語才是兩句的主詞，但不定詞片語當主詞時，會讓句子頭重腳輕，所以使用虛主詞it，讓不定詞片語成為delayed subject，句子較自然。
有的it + is的句子是因為is是當主要動詞，或當是助動詞（幫助主要動詞形成被動態，或進行式）。這不是本句的狀況。

Adverbial clauses may be reduced by changing them into
(1)single adverb----mistakenly
(2)prepositional phrase-------to be mistaken for
(3)infinitive phrase------to be mistaken
(4)oarticipial phrase------As Bob is to be mistakened for a girl, he went home.
----After he was mistakened for a girl, he was embrassed.------It is =He was can only be used here as the Adverb.Others are subsititued by prep.phrase; infinitive phr. participial phrase---