例如：

- That he is late is not surprising.（他迟到并不奇怪。）

- Whether we can finish the work on time is still uncertain.（我们能否按时完成工作还不确定。）

二、常见的引导词及用法

三、主语从句的常见句型

1. It is + 形容词/名词 + that 从句

- It is important that we should be on time.

- It is a fact that he is honest.

2. It is known / believed / said / reported that...

- It is known that he is a doctor.

- It is said that the meeting has been canceled.

3. It is no wonder that...

- It is no wonder that he failed the test.

4. It is (not) necessary / possible / likely that...

- It is necessary that we leave now.

- It is unlikely that they will come.

四、主语从句与形式主语“it”的使用

在英语中，为了使句子结构更平衡，常常使用形式主语“it”来代替真正的主语从句。这种结构被称为“it is + 形容词/名词 + that 从句”。

例如：

- It is obvious that he is lying.

- It is unclear whether she will join us.

这种结构避免了将较长的从句放在句首，使语言更自然流畅。

五、主语从句的注意事项

1. 引导词不能省略：当引导词是“that”时，一般不能省略，尤其是在正式文体中。

2. 主谓一致：主语从句作为整体，动词应使用第三人称单数形式。

- What he said is true.

3. 避免重复：避免在主语从句中重复使用“that”或其他引导词，除非必要。

六、总结表格

通过掌握主语从句的结构和用法，可以更准确地理解和运用复杂的英语句子，提升语言表达的层次感和逻辑性。