it is adj that，其中that引导的从句是主语从句；
it是形式主语，为了避免头重脚轻，才把主语从句放在后面。
举例说明：
1. It's strange that he should be late.
真奇怪，他竟会迟到。
正常的句子是：That he should be late is strange.
可以看出来系动词和表语很简短，为了可以很快了解句子的意思，用形式主语it代替that引导的主语从句。
分析：that he should be late主语从句作主语；is系动词作谓语；strange是表语。
2. It's important that we play well at home.
重要的是我们应该打好国内的比赛。
正常的句子是：That we play well at home is important.
分析：That we play well at home主语从句作主语； is系动词作谓语； important表语。
3. It's likely that we'll add a new division this year.
今年我们可能会新增一个事业部。
正常的句子是：That we'll add a new division this year is likely.
分析：That we'll add a new division this year主语；is系动词作谓语；likely是表语。