形式主语和强调句的区别？

It is difficult to answer the question.
It在句子中做形式主语，其真实主语是answer the question，即原句为Answer the question is difficult.用It做主语是为了保持句子平衡。

It was here that I came.这是强调句.
原句应为I came here.用强调句是为了强调here这个地方。