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	Definition	Example
after	time later than	After I finished, I went home.
before	time earlier than	Before she arrives, call me.
because	reason or cause	I stayed because it was important.
although	contrast with unexpected result	Although it rained, we played.
if	condition	If you study, you will improve.
so that	purpose or goal	I whispered so that no one heard.

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	Example
Use a comma after an introductory dependent clause	When I arrived, the meeting started.
Usually no comma when the dependent clause is last	The meeting started when I arrived.
Comma may appear with nonessential relative clauses	My car, which is old, still runs well.

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	Example
Essential clause identifies which one and usually no commas	The students who studied passed.
Nonessential clause adds extra info and uses commas	The students, who studied, passed.
That often introduces essential clauses	The book that you lent me was great.
Which often introduces nonessential clauses in careful writing	The book, which you lent me, was great.

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.