It was yesterday that he met my brother in the Zoo.我知道这个是强调句

It was yesterday that he met my brother in the Zoo.我知道这个是强调句,怎么把它变为表语从句?
It was yesterday that he met my brother in the Zoo.我知道这个是强调句,那它为什么不是表语从句呢?如果把它变为表语从句,是不是把that变为when啊?亦或是只能用作强调句?