A sentence is a group of words containing a subject and a predicate and expressing a complete thought. In order for this definition to be helpful, you must be able to recognize a subject and a predicate, and understand what is meant by “a complete thought.”
Subject and predicate
A sentence has a subject (what or whom the sentence is about) and a predicate. The predicate tells what the subject does or is or what is done to the subject (for example, The books were left outside). The simple subject is a noun or pronoun. The complete subject is this noun or pronoun and the words that modify it. The simple predicate is a verb or verb phrase (for example, has walked, will have walked). The complete predicate is the verb or verb phrase and the words that modify or complete it.
In the following example , Ron is both the simple and the complete subject of the sentence. Shot is the simple predicate. Shot Guido is the complete predicate: the verb shot and its direct object Guido.
Ron shot Guido.
In the following example, man is the simple subject. The angry man in pajamas is the complete subject. Stood is the verb; stood on the front porch is the complete predicate.
The angry man in pajamas stood on the front porch.
In the following sentence , argument is the simple subject modified by the adjective clause that money is a burden. The argument that money is a burden is the complete subject. The simple predicate is the verb originated. The rest of the sentence is the complete predicate.
The argument that money is a burden probably originated with a rich man who was trying to counter the envy of a poor man.
In the short, first sentence, the subject and predicate are easy to identify. In the second sentence, you can pick out the simple subject and verb fairly easily, despite the modifiers. But the third sentence is longer and more complicated. As you begin to write more sophisticated sentences, the simple subject and simple predicate may seem to get lost in a web of modifying words, phrases, and clauses. To ensure that you have a complete sentence, you still should be able to identify the core noun or pronoun and the core verb or verb phrase.
Expressing a complete thought
In addition to having a subject and predicate, a sentence must be able to stand on its own. It can't depend on something else to express a complete thought. Look at the following example.
This is a grammatically complete sentence, although it's not very interesting. It has a subject ( he) and a verb ( jumped). It expresses a complete thought. You know what happened. You might want to know more about the person—who he is or why he jumped, for example. You might want to know more about the jump itself—when it occurred, how high it was, and so on. But the basic action is complete: He jumped.
The next example is an incomplete sentence. It still has a subject ( he) and a verb ( jumped), but the word When keeps this group of words from being a complete thought: What happened when he jumped?
When he jumped.
The following sentence is still an incomplete sentence. Now, you know where he jumped, but the thought is still incomplete: What happened when he jumped high into the air?
When he jumped high into the air.
The next example is a complete sentence again. The question “What happened when he jumped?” has been answered: he looked as if he were flying. Even if the phrase high into the air were deleted, the thought would be complete.
When he jumped high into the air, he looked as if he were flying.
By varying sentence types in your writing, you will be able to control the pacing and clarity of the paragraphs. Using a variety of sentence types also makes for more interesting reading.
A simple sentence has one independent clause and no subordinate clauses.
Old‐growth forests in the United States are disappearing.
Citizens must act.
A compound sentence has two or more independent clauses, joined by coordinating conjunctions, and no subordinate clauses.
Old‐growth forests in the United States are disappearing, and citizens must act.(two independent clauses joined by and)
A complex sentence contains one independent clause and one or more subordinate clauses.
Because old‐growth forests in the United States are fast disappearing, citizens must act now. ( Because old‐growth forests in the United States are fast disappearing = subordinate clause beginning with subordinating conjunction; citizens must act now = independent clause)
Forests that have existed for thousands of years are in danger. ( that have existed for thousands of years = subordinate clause beginning with relative pronoun; Forests …are in danger = independent clause)
A compound‐complex sentence joins two or more independent clauses with one or more subordinate clauses.
Forests that have existed for thousands of years are in danger, and citizens must take action. ( Forests are in danger and citizens must take action = independent clauses; that have existed for thousands of years = subordinate clause)
In your writing, try to vary your sentence structures by making use of all these types of sentences. Don't string together a long series of simple sentences; on the other hand, don't always write compound and complex sentences. Try beginning with a simple sentence, or try following several long compound and complex sentences with a simple one. It can have a surprisingly forceful effect. Note the last two simple sentences in this paragraph.
Because America seemed to provide limitless natural resources, we spent them freely. We mined for minerals, diverted rivers, and cut down trees, many of which had been growing for thousands of years before the first settlers arrived. Over the years, America's wilderness has given way to prosperous cities, and skyscrapers have replaced giant old trees. America has succeeded. But now we are paying the price.
Sentence structure can enhance the topic or purpose of your writing. For example, short sentences with action verbs can accelerate the pace of a narrative essay, but you may need to use compound and complex sentences to compare and contrast ideas in an argument paper.
Active voice vs. passive voice
Use the active voice rather than the passive voice of the verb whenever possible. Sentences structured in active voice are more energetic and more concise. Look at the following examples of weak passives. Notice how the subjects and verbs determine the action in active and passive sentences:
A speech was given by the delegate from Michigan, and a challenge was issued by him to everyone attending.
BETTER The delegate from Michigan gave a speech and issued a challenge to everyone attending.
After the town was hit by a tornado, a call to the Red Cross was made by our mayor.
BETTER After a tornado hit the town, our mayor called the Red Cross.