It is 可用于强调句、定语从句、主语从句等多种句型，举例如下：
It is(was)that句型,It is+被强调成分+that+句子剩余部分（强调句）：
可以分别用来强调主语、宾语、地点状语和时间状语。如：
It was he who(that)read three books in the library yesterday.(强调主语)
是他昨天在图书馆读了三本书。
It was in the library that he read three books yesterday.(强调地点状语)
他昨天是在图书馆里读了三本书。
It is+名词+定语从句：
It was the day my grandmother was angry.
那一天我外婆生气了。
It is +名词/形容词+不定式主语：
It做形式主语，句子真正的主语是不定式。
It was a pleasure to draw pictures.
画画是一种享受。