Simple, Compound and Complex Sentences: Rules and Examples
Overview
Every sentence in English is built from clauses, and the number and type of clauses determine which structural category it belongs to. A clause is a group of words that contains a subject and a verb. Some clauses can stand alone as complete sentences. Others cannot. The relationship between these clauses is what separates simple sentences from compound ones, and both from complex sentences.
Knowing how to join clauses correctly, and which punctuation each joining method requires, prevents a wide range of common errors. A writer who commands all three sentence types can also control pace, emphasis, and the logical relationships between ideas in ways that a writer limited to one type cannot.
Simple Sentences
A simple sentence contains one independent clause. An independent clause has a subject and a verb and expresses a complete thought on its own.
Simple does not mean short. A simple sentence can carry a great deal of information through modifiers, prepositional phrases, and objects, as long as all of that information is contained within a single independent clause.
That sentence is still simple. It has one subject, one verb, and one complete thought despite its length.
Compound Subjects and Compound Verbs
A simple sentence can have more than one subject or more than one verb, provided there is still only one independent clause.
These remain simple sentences because they contain a single clause with no additional independent or dependent clause attached.
Compound Sentences
A compound sentence contains two or more independent clauses joined together. Each clause could stand alone as a complete sentence. They are connected by a coordinating conjunction, a semicolon, or a conjunctive adverb.
The seven coordinating conjunctions in English are for, and, nor, but, or, yet, and so, often remembered by the acronym FANBOYS. When a coordinating conjunction joins two independent clauses, a comma precedes it.
A semicolon can also join two independent clauses without a conjunction, provided the ideas are closely related.
Conjunctive Adverbs
Words like however, therefore, moreover, and nevertheless can connect two independent clauses. When they do, a semicolon precedes them and a comma follows them.
Using a comma alone before a conjunctive adverb, without a semicolon, produces a comma splice.
Complex Sentences
A complex sentence contains one independent clause and at least one dependent clause. A dependent clause has a subject and a verb but cannot stand alone as a sentence. It depends on the independent clause to complete its meaning.
Dependent clauses are introduced by subordinating conjunctions. Common subordinating conjunctions include because, although, when, while, if, since, before, after, unless, and even though.
Punctuation in Complex Sentences
The position of the dependent clause determines whether a comma is needed. When the dependent clause comes first, a comma separates it from the independent clause. When the independent clause comes first, no comma is required.
Comparing the Three Sentence Types
| Type | Clause Structure | Joining Method | Example |
|---|---|---|---|
| Simple | One independent clause | None required | She reads every evening. |
| Compound | Two or more independent clauses | Coordinating conjunction or semicolon | She reads every evening, and her brother watches films. |
| Complex | One independent clause plus one or more dependent clauses | Subordinating conjunction | She reads every evening because it helps her relax. |
A compound sentence presents two ideas as roughly equal. A complex sentence places one idea in a dependent position, signalling that it supports, explains, or qualifies the main idea.
In the compound version, both facts are presented side by side. In the complex version, the weather is given as the explanation for the road conditions. The choice between these structures is a choice about how to frame the relationship between ideas.
Common Mistakes to Avoid
Mistake 1: The Comma Splice
Joining two independent clauses with only a comma, without a coordinating conjunction or semicolon, is called a comma splice.
Mistake 2: The Run-On Sentence
A run-on sentence joins two or more independent clauses with no punctuation or conjunction at all.
Mistake 3: Treating a Dependent Clause as a Complete Sentence
A dependent clause introduced by a subordinating conjunction cannot stand alone. Writing it as a separate sentence produces a sentence fragment.
Mistake 4: Missing the Comma After a Leading Dependent Clause
When a dependent clause opens a complex sentence, a comma must follow it.
Mistake 5: Using a Comma Before a Subordinating Conjunction When the Independent Clause Leads
When the independent clause comes first in a complex sentence, no comma is needed before the subordinating conjunction.
Mistake 6: Confusing Coordinating and Subordinating Conjunctions
Coordinating conjunctions join equal clauses. Subordinating conjunctions introduce dependent clauses. Using one in place of the other changes the grammatical relationship between the ideas.
Practice Exercises
Exercise 1: Identify the Sentence Type
Read each sentence and write whether it is simple, compound, or complex.
- The library opens at nine and closes at eight.
- She did not attend the meeting because she was travelling.
- The manager reviewed the report, revised two sections, and sent it to the client.
- He finished early, so he offered to help his colleague.
- Unless the weather improves, the outdoor event will be moved indoors.
- The children played in the garden.
- The flight was delayed; the passengers waited in the terminal.
- After the presentation ended, the audience asked several questions.
Exercise 2: Join the Clauses
Join each pair of sentences using the word or punctuation shown in brackets. Rewrite as a single sentence.
- The report was incomplete. The manager returned it. (so)
- She studied all weekend. She did not feel prepared. (although)
- The café was full. They found a table near the window. (but)
- He sent the email. He did not receive a reply. (semicolon)
- The flight lands at noon. The driver will be waiting. (when)
Exercise 3: Correct the Mistake
Each sentence contains one error. Rewrite it correctly.
- The event was a success, the organisers were pleased.
- Because the traffic was heavy. She arrived twenty minutes late.
- He accepted the offer, although he needed time to consider it. (Rewrite so the contradiction is expressed more naturally.)
- They worked all night they finished the project before sunrise.
- Although she had studied hard, but she still found the exam difficult.
Summary
| Type | Structure | Key Joining Words | Punctuation Note |
|---|---|---|---|
| Simple | One independent clause | None | No joining punctuation needed |
| Compound | Two or more independent clauses | and, but, so, or, yet, for, nor | Comma before the conjunction; semicolon without |
| Complex | One independent clause plus one or more dependent clauses | because, although, when, if, since, unless | Comma after leading dependent clause; no comma when independent clause leads |
Simple sentences deliver clear, direct information. Compound sentences connect two equally weighted ideas. Complex sentences show how one idea depends on, explains, or qualifies another. Using all three with intention is what gives writing its shape and precision.