什么时候用it什么时候用that?

当需要强调句中的主语、宾语、状语时，我们常用“It is(was)+被强调部分+that(who)”这种句型，使用此句型时，应注意以下几点。

1.一般情况用It is(was)……that…如：
He read three books in the library yesterday.

我们分别可以强调主语、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)

It was three books that he read in the library yesterday.(强调宾语)

It was in the library that he read three books yesterday.(强调时间状语)

2.当被强调部分指人时，可用that，也可用who；指物时，只用that,如：

It was Tom who(that)I met last week.

It is a new book that his brother wants to buy.

3.强调状语时，只用that,不用when、where.如：

It is at 5 o'clock that the train will arrive.

4.被强调的部分是主语时，注意句子的谓语动词和被强调的主语保持一致。如：

It is he who is late. It is they that were late.

5.一般疑问句的强调句为“Is (was) it + that…?”；特殊疑问句为：“特殊疑问词+ is (was) +that…?”结构。如：

Was it ten years ago that his father died?（他的父亲是十年前去世的吗？）

When is it that you will set off?（