by Craig Shrives

What Is a Sentence?

A sentence is a group of words giving a complete thought. A sentence must contain a subject and a verb (although one may be implied). For example:
  • The cat sat on the mat.
  • (Here, the subject is "the cat" and the verb is "sat." The words convey a complete thought. This is a sentence.)
  • Eat!
  • (It's only one word, but this is also a sentence. In this example, the subject is implied. In full, it would be "you eat." Remember that a sentence must have a subject and verb, even if one is implied, and must express a complete thought.)

A More Formal Definition of Sentence

A sentence is a set of words that is complete in itself, typically containing a subject and predicate, conveying a statement, question, exclamation, or command, and consisting of a main clause and sometimes one or more subordinate clauses.

Oxford Dictionary

Table of Contents

  • The Four Types of Sentence
  • The Four Sentence Structures
  • (1) Simple Sentence
  • (2) Complex Sentence
  • (3) Compound Sentence
  • (4) Compound-Complex Sentence
  • Why Understanding Sentences Is Important
  • Video Lesson
  • Printable Test

The Four Types of Sentence

four types of sentence
A sentence can convey a statement, a question, an exclamation, or a command. There are four types of sentence:

(1) Declarative Sentence

A declarative sentence states a fact and ends with a period (full stop). For example:
  • He has every attribute of a dog except loyalty. (Politician Thomas P Gore)
  • I wonder if other dogs think poodles are members of a weird religious cult. (Comedian Rita Rudner)
  • (Remember that a statement which contains an indirect question (like this example) is not a question.)

(2) Imperative Sentence

An imperative sentence is a command or a polite request. It ends with an exclamation mark or a period (full stop). For example:
  • When a dog runs at you, whistle for him. (Philosopher Henry David Thoreau, 1817-1862)

(3) Interrogative Sentence

An interrogative sentence asks a question and ends with a question mark. For example:
  • Who knew that dog saliva can mend a broken heart? (Author Jennifer Neal)

(4) Exclamatory Sentence

An exclamatory sentence expresses excitement or emotion. It ends with an exclamation mark. For example:
  • In Washington, it's dog eat dog. In academia, it's exactly the opposite! (Politician Robert Reich)

The Subject Could Be Implied.

In an imperative sentence (an order) or an interrogative sentence (a question), the subject or verb is often implied.
  • Run!
  • Go.
  • (This is the shortest sentence in English.)
  • Why?
The shortest sentence without an implied subject or verb is "I am" or "I go."

The Four Sentence Structures

A sentence can consist of a single clause or several clauses. When a sentence is a single clause, it is called a simple sentence (and the clause is called an independent clause). A sentence must contain at least one independent clause. Below are the four types of sentence structure (with their independent clauses shaded):

(1) Simple Sentence

A simple sentence has just one independent clause. For example:
  • You can't surprise a man with a dog. (Screenwriter Cindy Chupack)

(2) Complex Sentence

A complex sentence has an independent clause and at least one dependent clause. For example:
  • Diplomacy is the art of saying "nice doggie" until you can find a rock. (Actor Will Rogers)
  • When you're on the Internet, nobody knows you're a dog. (Cartoonist Peter Steiner)

(3) Compound Sentence

A compound sentence has at least two independent clauses. For example:
  • Cry "Havoc," and let slip the dogs of war. (Playwright William Shakespeare)

(4) Compound-Complex Sentence

A compound-complex sentence has at least two independent clauses and at least one dependent clause. For example:
  • When a dog bites a man, that is not news because it happens so often, but if a man bites a dog, that is news. (Editor John B Bogart)
There are four great reasons to understand sentence structures and the types of the sentence.

(Reason 1) Avoid the run-on sentence.

By far the most common mistake made by people with otherwise sound writing skills is the run-on sentence. Typically, this error is caused by writing a sentence, putting a comma, and then writing another sentence.
  • I love the mountains, they remind me of home. wrong cross
  • Love is so short, forgetting is so long. wrong cross (Chilean politician Pablo Neruda)
You cannot end a sentence with a comma. These should both be two sentences (or rewritten to punctuate them correctly). Remember that a sentence contains a subject and a verb and gives a complete thought. The criteria for what constitutes a sentence are satisfied twice in each example.

The run-on sentence usually occurs because writers feel a period (full stop) is too much of speed bump between their closely related sentences. The jolt of a period can be smoothed with other punctuation (but not a comma). Here are some options:
  • Don't play hide and seek; no one would look for you.
  • (You can smooth the jolt of a period by merging your two sentences into one with a semicolon.)
  • I like a woman with a head on her shoulders – I hate necks. (Actor Steve Martin)
  • (You can smooth the jolt of a period by merging your two sentences into one with a dash. A dash looks quite stark, and it looks a little informal.)
  • My friend is a procrastinator...he's afraid of Saturday the 14th.
  • (You can smooth the jolt of a period by merging your two sentences into one with three dots (or ellipses). Using three dots creates a pause for effect, and it looks informal.)

(Reason 2) Punctuate your sentences correctly.

Understanding the four sentence structures assists with deciding how to punctuate sentences. More specifically, it assists with the following two common decisions:

(1) Deciding whether to use a comma with the subordinate clause in a complex sentence.

A complex sentence comprises an independent clause (shaded) and at least one subordinate clause. When the subordinate clause is at the front and acts like an adverb – typically stating a time (e.g., When I was six), a place (e.g., Where I live), or a condition (e.g., If I were you) – then it is a common practice to offset it with a comma. When such a clause appears at the back, it is usually not offset with a comma. Here are some examples:
  • When I was six, I had a wind-up Evil Knievel motorbike.
  • I had a wind-up Evil Knievel motorbike when I was six.
  • When you're on the internet,nobody knows you're a dog. (Cartoonist Peter Steiner)
  • Nobody knows you're a dog when you're on the internet.
Read more about adverbial phrases and adverbial clauses.

(2) Deciding whether to put a comma before a conjunction.

A compound sentence has at least two independent clauses (highlighted), which are usually joined with a conjunction (e.g., and, or, but). A conjunction (bolded) that joins two things is not normally preceded with a comma, but a conjunction that joins two independent clauses in a compound sentence is.
  • Lee likes pies and cakes.
  • (There is no comma before and. This is a simple sentence.)
  • Lee likes pies, and he likes cakes.
  • (This time, there is a comma before and. This is a compound sentence.)
  • Go, and never darken my towels again. (Comedian Groucho Marx)
  • (Remember that Go is the shortest sentence in English.)
Let's examine this point a little more. Look at these two examples:
  • I would say, "I'm alone, but I'm not lonely." correct tick (Actor Bruce Willis)
  • (Here, but is preceded with a comma because it's joining two independent clauses.)
  • I would say, "I'm alone but not lonely." correct tick
  • (Here, but is not preceded with a comma because it's joining two adjectives (alone and not lonely) not two independent clauses.)
Here's a tip: Look carefully for the subject and verb in the text after your conjunction to confirm the text is an independent clause. If it is, whack a comma in. If it isn't, don't use a comma.
  • Non-rabid wolves have attacked and killed people (mainly children), but this is rare. They live away from people and have developed a fear of humans from hunters and shepherds.
  • They live away from people, and they have developed a fear of humans from hunters and shepherds.
  • (Compare this compound sentence with the simple sentence (the last one) in the example above. When you add the word they after the and, the second half becomes an independent clause, and a comma is then required.)
Be aware that a compound sentence can have more than two independent clauses.
  • Some men are born mediocre, some men achieve mediocrity, and some men have mediocrity thrust upon them. (Playwright Joseph Heller)
  • (This is a compound sentence with three independent clauses. The first independent clause ends with just a comma. This is an occasion when that's allowable.)
  • "Veni, vidi, vici" [I came, I saw,I conquered.] (Roman emperor Julius Caesar)
  • (This is another occasion when you have to say it's acceptable to use just a comma to separate independent clauses (an error known as a run-on sentence or comma splice). Grammarians hate the comma splice so much, you will often see "Veni, vidi, vici" translated "As I came; I saw; I conquered" and even "I came, I saw, and I conquered.")
Read more about commas with conjunctions.

(Reason 3) As the subject of an imperative sentence is "you," you can't use "myself."

  • If you have any questions, email myself or your line manager. wrong cross
  • Please write to myself with any suggestions. wrong cross
The subject of an imperative sentence is "you," which is usually implied (i.e., not said or written). This means you cannot use "myself," which requires the subject to be "I." Writers often use "myself," believing it sounds more highbrow. It's wrong. It should be "me."

This is also covered in the entry on reflexive pronouns.

(Reason 4) Don't use a question mark with a declarative sentence that includes an indirect question.

  • She asked whether I loved her? wrong cross
  • I wonder if other dogs think poodles are members of a weird religious cult? wrong cross (Comedian Rita Rudner)
The bolded texts are indirect questions. These are declarative sentences (i.e., statements) not questions. They should end in periods (full stops).

Here is a 16-minute video summarizing this lesson on sentences.

Are you a visual learner? Do you prefer video to text? Here is a list of all our grammar videos.

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