在英语学习中，复合句是一个非常重要的语法知识点。它不仅能够丰富我们的表达方式，还能帮助我们更好地组织思想和信息。今天，我们就来通过一些具体的练习题来加深对复合句的理解，并附上详细的答案解析。

练习题一：选择正确的连接词

1. She was tired _______ she decided to take a break.

A. because B. so C. but D. or

答案解析：正确答案是A. because。句子的意思是“她很累，所以决定休息一下”，这里需要一个表示因果关系的连接词。

2. I will go to the park _______ it rains.

A. unless B. if C. although D. since

答案解析：正确答案是A. unless。句子的意思是“除非下雨，否则我会去公园”。

练习题二：改写句子为复合句

1. The teacher spoke clearly. We understood everything.

改写为复合句：

答案解析：The teacher spoke clearly so that we could understand everything. 这里使用了so that引导的结果状语从句。

2. It is raining. We have to stay at home.

改写为复合句：

答案解析：Because it is raining, we have to stay at home. 这里使用了because引导的原因状语从句。

练习题三：根据提示完成句子

1. Although he was very busy, _______ (he still helped his friend).

答案解析：Although he was very busy, he still helped his friend. 这里使用了although引导的让步状语从句。

2. She didn't come to the party _______ (she had an important meeting).

答案解析：She didn't come to the party because she had an important meeting. 这里使用了because引导的原因状语从句。

通过以上练习题，我们可以看到复合句在实际应用中的多样性和灵活性。希望大家能够在日常学习中多加练习，逐渐掌握复合句的使用技巧。记住，复合句不仅仅是语法的一部分，更是提高语言表达能力的有效工具。