从句（subordinate clause）是有自己的主语和动词、但 不能独立成句 的一组词。它为主句补充原因、时间、条件、对比或额外信息。用得好，你可以在一个句子里放进很多细节，而不丢掉重点。
这一课属于句子控制（sentence control）。它在 directed writing 和描写任务中特别有用，因为这些任务要求你表现想法之间的关系，而不只是把想法列出来。
从句有什么作用？
每个从句都由一个连接词带头，例如 because, although, when, while, if, unless, since, after, which, who 或 that。这个词告诉读者额外的细节与主要意思如何相连。
比较两个版本。“The team lost. They trained every day.” 让读者自己猜两者的关系。
“Although the team trained every day, they lost.” 在一个句子里就说清了这个对比。
技巧在于：把主要意思放进主句，把辅助细节放进从句。读者应该能划出核心陈述，并很快找到它。
怎样一步步构造句子？
- 先把主要意思写成简单句。 问自己：“如果只能保留这一点，读者需要什么？”
- 列出额外细节。 选一到两个，用来补充原因、条件或对比。
- 为每种关系选对连接词。 原因用 because，对比用 although，时间用 when。
- 把从句放在读起来最清楚的位置。 放在句首，它交代背景；放在句末，它补充说明。
- 检查主要意思是否仍然容易找到。 如果需要去找，就简化。
例题示范
三个短句：“Amir missed the start of the talk. He stayed until the end. He had promised to help clear the hall.”
步骤 1，主要意思。 重点是 Amir 留到了最后。
步骤 2，额外细节。 他错过了开头（对比），而且他答应过帮忙（原因）。
步骤 3，连接词。 对比用 although，原因用 because。
步骤 4，组合：
Although Amir missed the start of the talk, he stayed until the end because he had promised to help clear the hall.
步骤 5，检验。 主句 “he stayed until the end” 在中间，很容易找到。两个从句分列两侧，各有明确的任务。
常见错误
常见的错误是把从句一层叠一层，直到主要意思被埋住。
错误： The festival, which was held in the park that the council had recently cleaned after the floods that had closed the road, attracted crowds, although the weather, which had been poor all week, was cold.
这个句子有五个从句，主要意思 “the festival attracted crowds” 藏在中间。
改正方法是选出重要的细节，把其余的移到另一个句子：
The festival attracted large crowds, although the weather was cold. It was held in the park that the council had cleaned after the floods.
现在每个句子都只有一个主要意思和一个辅助细节。
第二个较小的错误，是把从句当成句子来标点：“Because the bus was late.” 这是残句。把它接到主句上：“Because the bus was late, Hana walked.”
自我检测
1. 划出主句：“When the lights went out, the audience waited quietly until the stage manager spoke.”
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主句是 “the audience waited quietly”。“When the lights went out” 和 “until the stage manager spoke” 都是从句，因为它们不能独立成句。
2. 合并成一个主要意思清楚的句子：“Mei Ling was nervous. She volunteered to go first. She wanted to get it over with.”
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一种好的写法：“Although Mei Ling was nervous, she volunteered to go first because she wanted to get it over with.” 主要意思是她自愿第一个上。用 “Since” 表示原因的版本也可以接受。
3. 修正残句：“The team celebrated. Because they had finally won.”
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把两部分连起来：“The team celebrated because they had finally won.” 或者把从句放在前面：“Because they had finally won, the team celebrated.” 从句放在后面时不需要逗号。
接下来学什么
长句还有另一个风险：描述性短语可能挂到错误的词上。澄清有歧义的修饰语教你怎样修正。你可以回顾修正逗号连接句中的连接方式，并用句子控制练习测试整个单元。
写作修订对比视图可以检查修改后主要意思是否更容易找到，摘要筛选与改写工作室则帮你决定保留哪些细节。
如果你总是在添加细节时丢掉重点，老师和你一起读草稿，就能指出让句子变清晰的那一刀，这是线上一对一 English First Language 补习的一部分。