最简单易懂的讲解，

例如，Danny broke the window yesterday.

强调Danny ---- It is Danny who broke the window yesterday.

强调yesterday----It is yesterday that Danny broke the window.

特点： 被强调的部分原本是后面句中的某个成分。被提到前面加以强调了。

It was my suggestion that made him confident of success. 这就是强调句。

正是我的建议使他对成功有了自信。

It is my suggestion that he needs more practice.

他需要更多的练习是我的建议

最后这句就是主语从句，it 指代后面的that he needs more practice作主语。

my suggestion 不是that he needs more practice中的一个成分。