it is ______that/who
当去掉“it is that/who”仍可成为一句话,那么就是强调句
例如：It was in the park that they had a picnic yesterday.
可变为：They had a picnic in the park yesterday.
我觉得这种方法是最容易记住的,比那些语法里的术语要好记,当然你也可以看看语法书.