在英语语法中，主语从句是一种重要的句式结构，它充当句子的主语，表达一个完整的思想或陈述。为了帮助大家更好地理解和掌握这一知识点，以下是一些精心设计的专项练习题。通过这些题目，大家可以进一步熟悉主语从句的构成和使用方法。

一、选择正确的主语从句

1. _______ that he will come tomorrow is true.

A) What

B) That

C) Who

D) Which

2. _______ we should do next remains a mystery.

A) Where

B) When

C) What

D) How

3. It is important to know _______ the meeting will be held.

A) when

B) where

C) why

D) what

4. _______ she said at the conference surprised everyone.

A) That

B) What

C) How

D) Why

5. _______ he is doing now is not clear to us.

A) What

B) Which

C) Where

D) Why

二、填空题

6. _______ you believe in yourself is the key to success.

7. _______ they will arrive is still unknown.

8. _______ the teacher explained made sense to me.

9. _______ book you borrowed from the library needs to be returned soon.

10. _______ the future holds is always exciting.

三、改写句子

将以下句子改为含有主语从句的形式：

11. His arrival was unexpected.

改为：_________

12. The fact that he was late annoyed everyone.

改为：_________

13. Whether we win or lose doesn’t matter.

改为：_________

14. She told me the reason for her absence.

改为：_________

15. They discussed the plan last night.

改为：_________

四、翻译练习

16. 他是否能按时完成任务尚不清楚。（用主语从句翻译）

________________

17. 我们不知道明天是否会下雨。（用主语从句翻译）

________________

18. 她刚才说的话让我感到困惑。（用主语从句翻译）

________________

通过以上练习，希望大家能够更加熟练地运用主语从句。主语从句不仅在语法上具有重要意义，而且在实际应用中也非常常见。希望同学们在学习过程中多加思考，不断巩固自己的知识体系！