有个句型it+be+形容词+ to do sth.这时It是形式主语,to do sth是真正主语,而这个句式可以用于某些动词后面,形成宾语从句,只要把be去掉就可以转化成简单句,如：We find it is difficult to finish the work within 2 hours= We find it difficult to finish the work within 2 hours ,简单句中It叫形式宾语,形容词由表语变为宾补,不定式由真正主语变成真正宾语,所以如果不定式做宾语,形式宾语it不加is
