A complex sentence has one independent clause that can stand alone and one or more dependent clauses that cannot stand alone. The dependent clause adds extra information such as time, reason, contrast, condition, or purpose. Complex sentences help you show clear relationships between ideas instead of listing short sentences.

An independent clause is a complete idea with a subject and a verb, and it can be a full sentence by itself. In a complex sentence, it carries the main message. The dependent clause supports it by answering questions like when, why, or under what condition.

A dependent clause also has a subject and a verb, but it does not express a complete thought on its own. It must be connected to an independent clause so the reader understands the full meaning. Dependent clauses often begin with a subordinating conjunction or a relative word.

Subordinating conjunctions introduce many dependent clauses and show the relationship to the main clause. They can express time, cause, contrast, condition, concession, purpose, or result. Choosing the right conjunction makes the logical connection between ideas precise.

Word/Phrase	🇨🇳 中文 词/短语	Definition	🇨🇳 中文 定义	Example	🇨🇳 中文 示例
after	⏱️ 之后	time later than	⏱️ 比…晚的时间	After I finished, I went home.	⏱️ 在我完成后，我回家了。
before	⏱️ 之前	time earlier than	⏱️ 比…早的时间	Before she arrives, call me.	⏱️ 在她来之前，给我打电话。
because	🧠 因为	reason or cause	🧠 原因或原因	I stayed because it was important.	🧠 我留下来，因为这很重要。
although	⚖️ 虽然	contrast with unexpected result	⚖️ 对比，结果出人意料	Although it rained, we played.	⚖️ 虽然下雨，我们还是玩耍了。
if	✅ 如果	condition	✅ 条件	If you study, you will improve.	✅ 如果你学习，你就会进步。
so that	🎯 以便	purpose or goal	🎯 目的或目标	I whispered so that no one heard.	🎯 我小声说，以便没有人听见。

A dependent clause can come before or after the independent clause. Putting the dependent clause first often highlights the condition or background information, while putting it second keeps the main idea first. The meaning stays similar, but the focus and flow can change.

Use a comma when a dependent clause comes before the independent clause. Do not usually use a comma when the dependent clause comes after the independent clause, unless another rule requires it. Correct comma placement helps readers see where the introductory dependent clause ends.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Use a comma after an introductory dependent clause	🧩 在引导性从属从句之后使用逗号	When I arrived, the meeting started.	🧩 当我抵达时，会议开始。
Usually no comma when the dependent clause is last	🧩 从属从句在句末时通常不使用逗号	The meeting started when I arrived.	🧩 会议在我到达时开始。
Comma may appear with nonessential relative clauses	🧩 非必要的关系从句可能使用逗号	My car, which is old, still runs well.	🧩 我的车子，旧的那辆，仍然运行良好。

Relative clauses are dependent clauses that describe a noun. They often start with who, whom, whose, which, or that, and they give identifying or extra information. Learning the difference between essential and nonessential relative clauses helps you choose commas correctly and avoid changing meaning.

An essential relative clause is necessary to identify which person or thing you mean, so it usually has no commas. A nonessential relative clause adds extra information about something already identified, so it is set off with commas. This choice can change meaning, not just punctuation.

Rule	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Essential clause identifies which one and usually no commas	🎯 必要从句用于识别对象，通常没有逗号	The students who studied passed.	🎯 学生中学习的那组通过了。
Nonessential clause adds extra info and uses commas	➕ 非必要从句添加额外信息并使用逗号	The students, who studied, passed.	➕ 学生们，学习的那组，通过了。
That often introduces essential clauses	🧩 That 常用于引入必需从句	The book that you lent me was great.	🧩 你借给我的那本书很棒。
Which often introduces nonessential clauses in careful writing	🧩 Which 常用于在谨慎写作中引入非必需从句	The book, which you lent me, was great.	🧩 你借给我的那本书很棒。

Complex sentences are especially useful for showing clear logical relationships. Time clauses show when something happens, reason clauses explain why, condition clauses show what must happen first, and concession clauses show contrast despite expectations. Understanding the relationship helps you select the best connector and keep your writing precise.

Keep complex sentences clear by making sure the dependent clause clearly attaches to the right idea. Avoid very long chains of dependent clauses that hide the main point. Use complex sentences to vary rhythm, show logic, and improve flow, but keep the main clause easy to find.