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状语从句是英语语法中一个非常重要的概念，通常用来描述一个主句中的动作或状态。状语从句通常由从属连词引导，从而将其与主句相连。
状语从句主要分为三类：时间状语从句，条件状语从句和原因状语从句。每一类从句都有其独特的语法规则和用法。
时间状语从句
时间状语从句用来描述主句中的动作或状态发生的时间。常用的从属连词有：when，while，before，after，since，until等等。例如：
- When I arrived at the school, the class had already started.
- While I was studying for my exam, my phone rang.
- Before she left the house, she checked her bag.
条件状语从句
条件状语从句用来描述一个条件，以便确定主句中的动作或状态是否会发生。常用的从属连词有：if，unless，provided that，as long as等等。例如：
- If it rains tomorrow, we will stay at home.
- Unless you finish your homework, you cannot watch TV.
- Provided that you arrive on time, we will begin the meeting.
原因状语从句
原因状语从句用来描述主句中的动作或状态的原因。常用的从属连词有：because，since，as，for等等。例如：
- Because it was raining outside, we decided to watch a movie.
- Since he had a headache, he didn't go to work today.
- As it was getting late, we decided to leave the party.
总之，状语从句是英语语法中至关重要的一部分。学会使用状语从句可以使你的语言更加流畅，更富有表现力。
