英文it虛主詞(Expletive it in English)

It可當虛主詞來代替較長的不定詞片語或名詞子句

不定詞片語	It+be動詞+形容詞+(for+人)+to+V It is hard for Jack to finish the homework.(對Jack來說，完成功課是困難的)
不定詞片語	It+be動詞+形容詞+(of+人)+to+V It is kind-hearted of you to help others.(你幫助其他人，真是善良)
名詞子句	It+be動詞+形容詞+that+子句 It is very tiring that you work day and night.(你日夜工作是很累人的)

※虛主詞選用for時，形容詞修飾｢事物｣，而選用of，則是形容詞修飾｢人｣