Complex Sentences: Definition, Subordinators, Comma Rules and Examples

A complex sentence contains one independent clause (a complete thought) and at least one dependent clause that cannot stand alone, joined by a subordinator such as because, although, when or who. Example: "Although the library was crowded, Maya finished her essay." Put a comma after an opening dependent clause, and usually none before a closing one.

What is a complex sentence?

A complex sentence is a sentence with one independent clause and at least one dependent clause. The independent clause could stand alone as a sentence. The dependent clause cannot, because it begins with a word that leaves the thought unfinished.

Because the train was late, the meeting started without us.

"Because the train was late" is the dependent clause. Read alone, it leaves you waiting for the result. "The meeting started without us" is the independent clause: it makes complete sense by itself. Put together, they form a complex sentence.

The word "complex" describes the grammar, not the difficulty. "I stayed home because it rained" is a complex sentence even though a child could say it.

Independent and dependent clauses

A clause is a group of words with a subject and a verb.

A dependent clause is also called a subordinate clause. If you write one on its own, you create a fragment: "Although she studied all night." The reader expects an ending. For more on the difference, see independent vs dependent clauses.

The three kinds of dependent clause

Dependent clauses do a job inside the sentence, and it helps to know which job.

• Adverb clauses say when, why, how, where or under what condition something happens. "We left when the rain stopped."
• Adjective (relative) clauses describe a noun. "The student who wrote the best draft presented first."
• Noun clauses act as a subject or object. "What the survey shows is surprising." / "She explained that the deadline had moved."

Most textbook exercises on complex sentences focus on adverb clauses, because the subordinators are the easiest to spot.

Subordinators: the words that start dependent clauses

A subordinating conjunction turns a full sentence into a dependent clause. Learn the groups, not a random list.

Two notes. Words such as "since" and "while" belong to more than one group, so check the meaning in context. And a coordinating conjunction (for, and, nor, but, or, yet, so) does not make a complex sentence; it joins equal clauses and gives a compound sentence. See compound sentences and coordinating conjunctions for the contrast.

Comma rules for complex sentences

Placement of the dependent clause decides the punctuation.

1. Dependent clause first: use a comma. "When the bell rang, the students left."
2. Dependent clause last: usually no comma. "The students left when the bell rang."
3. Contrast clauses at the end can take a comma when they add a sharp contrast: "She accepted the offer, although the salary was lower."
4. Non-essential relative clauses take commas; essential ones do not. "My sister, who lives in Leeds, is a nurse" (you have one sister, and the clause adds extra information). "The sister who lives in Leeds is a nurse" (the clause identifies which sister).
5. "That" clauses are essential and take no comma. "The report that she sent was late."
6. Never put a comma between a subject and its verb, even when the subject is a long noun clause: "What the committee decided surprised everyone."

For the wider picture, see comma rules.

Examples from essays and everyday writing

Complex sentences carry reasoning, which is why essays rely on them.

Argument and reason

• Public libraries remain valuable because they give free access to quiet study space.
• Unless local councils protect their funding, many branches will cut their opening hours.

Concession and contrast

• Although online lectures are convenient, many first-year students say they learn better in person.
• Remote work saves commuting time, whereas office work makes spontaneous questions easier.

Cause and effect

• The experiment was repeated since the first result could not be explained.
• When the temperature rose above 30°C, the samples began to degrade.

Relative and noun clauses

• The author, whose earlier book won a national prize, argues that cities should plant more trees.
• Reviewers noted that the sample was small.

Workplace

• If you send the invoice before Friday, accounts can pay it in the next run.
• Please call me as soon as the client confirms.

Practice: combine simple sentences into complex ones

Take two related simple sentences and choose a subordinator that shows the relationship. Try the exercises first, then compare with the suggestions.

1. The bus was delayed. We missed the lecture.
2. She revised her thesis statement. Her tutor had questioned it.
3. The campus café closes at six. Students can still order takeaway.
4. The data were incomplete. The team published a cautious summary.
5. Priya wants a scholarship. She must submit the form by March.

Suggested answers (others are possible)

1. We missed the lecture because the bus was delayed.
2. She revised her thesis statement after her tutor questioned it.
3. Although the campus café closes at six, students can still order takeaway.
4. Because the data were incomplete, the team published a cautious summary.
5. If Priya wants a scholarship, she must submit the form by March.

Notice that each answer shows a relationship (cause, time, contrast, condition) that the two separate sentences left to the reader's guess. That is the real purpose of subordination.

Common mistakes

• Fragments. "Because the results were unclear." Fix: attach it to an independent clause, or remove the subordinator.
• Comma splices and run-ons. "The results were unclear, we repeated the test." Fix: add a subordinator ("Because the results were unclear, we repeated the test") or use a semicolon.
• Double conjunctions. "Although it was late, but we kept working." Use one connector: either "Although it was late, we kept working" or "It was late, but we kept working."
• Dangling modifiers. "While reading the report, the figures confused me." As written, the figures seem to be doing the reading. Write "While I was reading the report, the figures confused me."
• Overlong chains. Three or four dependent clauses in one sentence tire the reader. Split the sentence when you lose track of the main point.

How to use complex sentences well

Variety matters more than complexity. A page of only short simple sentences sounds choppy; a page of only complex sentences sounds heavy. A good rhythm mixes them, as in sentence structure, where simple, compound, complex and compound-complex sentences work together. Lead with the dependent clause when you want to set up context, and end with it when the main point should come first. Keep the subject and verb of the independent clause close together so the reader finds the main idea quickly.

Using AdeptWrite to practise and check

The Grammar Checker flags fragments, comma errors and agreement problems in your draft and shows each suggestion so you can accept or reject it. Treat it as a second reader, not a verdict: you decide whether a flagged clause is intentional. The Sentence Rewriter can tidy a stiff or tangled sentence, which is useful when a chain of clauses has become hard to follow. Each tool handles up to 6,000 characters per run, so split a long essay into sections.

Quick checklist

1. Does each sentence have at least one independent clause?
2. Does every dependent clause begin with a subordinator or relative word?
3. Did I put a comma after an opening dependent clause?
4. Did I avoid a comma before a closing essential clause?
5. Did I avoid "although ... but" and similar doubled connectors?
6. Can the reader find the main idea quickly, or should I split the sentence?

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