其基本句型是：“It + is/was + 被强调的成分 + that/who + 其她成分”。It 没有实意，只起语法作用，引导被强调的部分。当被强调的是人时，可用who(m)/that, 其他情况用that。
It was I who/that met Jack yesterday. 是我昨天碰到了Jack.(强调主语)
It was Jack that/whom I met yesterday. 我昨天碰到的是Jack。(强调宾语)
It was yesterday that I met Jack. 是昨天我碰到了Jack.(强调时间状语)
这几句话复原为非强调句就是：I met Jack yesterday.
It is people, not things, that are decisive. 决定的因素是人，不是物。(强调主语)
It is because the book is very important for my present job that I bought it. 是因为这本书对我目前的工作很有用，我才买了它。(强调原因状语)
It was in the supermarket that I gave the book to him. 是在那家超市里我给了他那本书。(强调地点状语)
1.强调主语：It was John who broke the window. 是John打破了窗子。
原句：John broke the window.
It is this overpass that will be pulled down. 将被拆掉的是这个天桥。
原句：This overpass will be pulled down.
It is the people who/that are really powerful. 真正有力量的是人民。
原句：The people are really powerful.
It was at that moment that he changed his mind.是在那一刻他改变了主意。
It is every day that Professor Smith goes swimming.史密斯教授是天天去游泳。
It was not until Saturday that he began to prepare for the examination. 他直到星期六才开始为考试作准备。
注意：这句话的原句是He did not begin to prepare for the examination until Saturday. 变成强调时间状语，注意 “not” 位置的变化。
It was in the library that I met Jack yesterday.是在图书馆我昨天碰到了Jack.
原句：I met Jack in the library yesterday.
It might have been on the bus that I lost my purse.可能是在公共汽车上我丢失了钱包。
原句是：I might have lost my purse on the bus. 强调地点状语，
也可改为：It was on the bus that I might have lost my purse.
It was under the tree that I was sitting then. 当时我正坐在那棵树下。