It is 被强调部分+ that + 句子其他部分。
It is from the sun that we get light and heat.
It was not until I had read your letter that I understood the true state of affairs.
He read three books in the library yesterday.
It was he whoread three books in the library yesterday.
It was three books that he read in the library yesterday.
It was in the library that he read three books yesterday.
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 whoI met last week.
It is a new boke that his brother wants to buy.
强调状语时, 只用that, 不用when、where. 如:
It is at 5 oclock that the train will arrive.