The Building Block: Clauses
Every sentence is built from clauses, so before examining sentence types, it helps to understand what a clause is.
A clause is a group of words that contains a subject and a verb. There are two kinds:
- An independent clause (also called a main clause) expresses a complete thought and can stand alone as a sentence. Example: The dog barked.
- A dependent clause (also called a subordinate clause) contains a subject and verb but does not express a complete thought on its own. It needs to attach to an independent clause. Example: because the stranger approached
The combination of independent and dependent clauses determines which sentence type you have. Understanding run-on sentences also depends on knowing how to handle these clause combinations correctly.
Simple Sentences
A simple sentence contains exactly one independent clause and no dependent clauses.
That sounds minimalist, but simple sentences are not necessarily short or plain. They can carry a long subject, a long predicate, or multiple phrases, as long as there is only one independent clause.
Basic Structure
[Subject] + [Verb] + [Optional: Object or Complement]
Examples
- She writes.
- The tall woman in the blue coat smiled at the camera.
- After a long night, the engineers finally fixed the bug.
- Marcus and Tia drove to the coast and watched the sunrise.
The last two examples have prepositional phrases and compound predicates, but there is still only one subject-verb relationship forming the core of each sentence.
Compound Subjects and Predicates
A simple sentence can have a compound subject (two subjects sharing one verb) or a compound predicate (one subject with two or more verbs), and it is still a simple sentence:
- Compound subject: The manager and the assistant reviewed the draft.
- Compound predicate: She opened the file and read the report.
When to Use Simple Sentences
Simple sentences are best for emphasis. A short, direct sentence after a series of longer ones draws attention to its content. They also improve readability when the information is genuinely straightforward and does not need qualification.
Compound Sentences
A compound sentence contains two or more independent clauses joined together. There are no dependent clauses.
The independent clauses can be joined in three ways: with a coordinating conjunction, a semicolon, or a semicolon followed by a conjunctive adverb.
Structure 1: Coordinating Conjunctions
The coordinating conjunctions are for, and, nor, but, or, yet, and so (often memorized as FANBOYS). Place a comma before the conjunction when joining two complete independent clauses:
- The team worked late, but they finished the project.
- She ordered coffee, and he ordered tea.
- You can take the highway, or you can take the back roads.
Structure 2: Semicolons
A semicolon joins two independent clauses that are closely related in meaning, without any conjunction:
- The report was overdue; the client was not happy.
- She practiced every day; her skills improved steadily.
The semicolon implies a logical connection between the two clauses. If you need to make that connection explicit, use a conjunctive adverb.
Structure 3: Semicolon + Conjunctive Adverb
Conjunctive adverbs (however, therefore, moreover, consequently, furthermore, meanwhile) connect clauses and show the logical relationship between them. They are preceded by a semicolon and followed by a comma:
- The deadline was tight; however, the team delivered on time.
- She studied abroad for a year; consequently, her language skills improved dramatically.
When to Use Compound Sentences
Compound sentences work well when two ideas are equally important and closely related. They suggest balance or contrast between the two clauses. Too many compound sentences in a row can make writing feel flat because both halves receive equal weight. Mix them with complex sentences for variety.
Complex Sentences
A complex sentence contains one independent clause and at least one dependent clause. The dependent clause cannot stand alone; it modifies or elaborates on the independent clause.
Dependent clauses are introduced by subordinating conjunctions or relative pronouns.
Common Subordinating Conjunctions
These words introduce dependent clauses: although, because, since, when, while, before, after, if, unless, until, as, even though, wherever, whether, though, once, as soon as.
Examples
- Although the weather was cold, they walked to the meeting.
- She did not reply because she had not seen the message.
- When the presentation ended, the audience applauded.
- He will not leave until the work is done.
Relative Clauses
Relative clauses are a specific type of dependent clause introduced by relative pronouns: who, whom, whose, which, that. They modify a noun in the independent clause:
- The engineer who designed the bridge received an award.
- The book that she recommended was sold out everywhere.
- We toured the factory where the parts are assembled.
Comma Placement in Complex Sentences
When the dependent clause comes first, place a comma after it before the independent clause:
- Because the data was incomplete, the team delayed the report.
- If you have questions, send them before Thursday.
When the independent clause comes first and the dependent clause follows, no comma is needed for most subordinating conjunctions:
- The team delayed the report because the data was incomplete.
- Send them before Thursday if you have questions.
An exception: when the dependent clause expresses contrast (using although, even though, though), many writers add a comma even when the dependent clause follows: She accepted the offer, although it was lower than expected.
When to Use Complex Sentences
Complex sentences are ideal for showing relationships between ideas: cause and effect, time sequences, conditions, and contrast. They add nuance that simple or compound sentences cannot. Academic and professional writing tends to rely heavily on complex sentences because they allow for qualification and precision.
Compound-Complex Sentences
A compound-complex sentence contains two or more independent clauses and at least one dependent clause. It combines the structures of both compound and complex sentences.
Examples
- Although the deadline was approaching, she stayed focused, and she submitted the project on time.
(Dependent clause + Independent clause + Independent clause) - He had not prepared for the interview, so he was nervous when he walked into the room.
(Independent clause + Independent clause joined by so + Dependent clause) - The proposal that she wrote was excellent, but the committee tabled it until next quarter.
(Independent clause with relative clause + Independent clause + Dependent clause)
When to Use Compound-Complex Sentences
These sentences can carry a lot of information efficiently, but they require careful construction. They work best when multiple related events or conditions form a single logical unit. Overuse or poor punctuation can make them confusing. If a compound-complex sentence runs longer than about 40 words, consider breaking it into shorter sentences.
Summary Table
| Sentence Type | Independent Clauses | Dependent Clauses | Example |
|---|---|---|---|
| Simple | 1 | 0 | The cat slept. |
| Compound | 2 or more | 0 | The cat slept, and the dog played. |
| Complex | 1 | 1 or more | Because it was warm, the cat slept outside. |
| Compound-Complex | 2 or more | 1 or more | Because it was warm, the cat slept outside, and the dog played in the yard. |
Why Sentence Variety Matters
Monotone sentence structure, where every sentence follows the same pattern, makes writing dull even when the content is interesting. Research on readability consistently finds that mixing sentence lengths and structures keeps readers engaged.
A common pattern in effective prose: a long complex or compound-complex sentence followed by a short simple sentence. The short sentence delivers the key point with impact after the buildup.
If you are revising a piece of writing and it feels flat, read it aloud. When every sentence lands with the same rhythm and length, that is the problem. Rephrasely can help you rephrase sentences to vary structure while preserving your meaning.
Sentence Types and Active vs. Passive Voice
Sentence type is distinct from active and passive voice. Any of the four sentence types can be written in either voice. A simple sentence can be passive: The report was written by Maria. A complex sentence can be active: Because she worked late, Maria finished the report.
Both considerations matter for clarity. Active voice and varied sentence structure together produce writing that is direct and easy to follow.
Sentence Types and Punctuation
Compound sentences require careful punctuation to avoid common errors:
- A comma splice joins two independent clauses with only a comma, without a coordinating conjunction. This is incorrect: She studied hard, she passed the exam. Fix: use a conjunction, semicolon, or period.
- A fused sentence (run-on) joins two independent clauses with no punctuation at all. See the full guide on run-on sentences for how to identify and fix these.
Comma rules also affect how complex and compound-complex sentences are punctuated. When in doubt, identify the clause boundary first and then decide whether a comma, semicolon, or conjunction is appropriate.
Frequently Asked Questions
What is the most common sentence type in everyday writing?
Complex sentences are by far the most frequent in both written and spoken English. They allow writers to show the relationship between a main idea and a qualifying or explanatory clause, which is a constant need in communication.
Can a simple sentence be long?
Yes. A simple sentence can be quite long if it has multiple phrases, a compound subject, a compound predicate, or many modifiers, as long as it contains only one independent clause. The experienced project manager and her team of ten engineers from three different departments worked through the weekend to deliver the final prototype ahead of schedule. That is still a simple sentence.
What is the difference between a phrase and a clause?
A phrase is a group of related words that does not contain both a subject and a verb. A clause does. In the morning is a phrase (no subject-verb pair). When morning arrives is a clause (subject: morning, verb: arrives).
Do all sentences have to have a subject and a verb?
In standard grammar, yes. Every complete sentence needs a subject and a finite verb. Imperative sentences appear to lack a subject (Sit down. Call me.), but the subject "you" is understood. One-word or short fragment responses are acceptable in conversation and fiction but avoided in formal writing.
How do I identify the independent clause in a complex sentence?
Remove the subordinating conjunction and its clause. What remains should express a complete thought. In Although the train was late, she made the meeting on time, removing Although the train was late leaves she made the meeting on time, which is complete. That is the independent clause.
Is it wrong to start a sentence with "because"?
No. Starting a sentence with because is grammatically correct if the dependent clause is followed by an independent clause: Because she prepared thoroughly, the presentation went well. The error occurs when because introduces a standalone fragment: Because she prepared thoroughly. That is incomplete. But starting a sentence with the conjunction is not itself a rule violation.