it作形式主语的例子有：It is+形容词＋to do sth .意思是做某事怎么样。这时it 是形式主语，不定式to do sth.是真正的主语。如：It is kind to help old people cross the street.。

It作形式宾语的例子有：I find it +形容词+to do sth.意思是我发现做某事怎么样…如I find it useful to water the flowers every day