Sentence fragments are not sentences because they do not express a complete thought. Some fragments also lack a subject or a verb. Some writers inadvertently include them in their writing because they either are writing in a hurry or aren’t aware that they’ve failed to include a subject or verb.
For a sentence to be a complete sentence and not a fragment, it must include a subject. Sometimes phrases can look like sentences but are still missing subjects. For example, the phrase "walking down the long, windy street" has a lot of words, but it is missing a subject. Who or what is walking down the street? This is a phrase that doesn’t express a complete thought or have a subject, and therefore, is a fragment. Make sure each sentence you write has a subject.
A similar situation arises when there is no verb in a phrase: "The beautiful but poorly dressed woman and her friend." This leaves the reader to wonder what the beautiful lady and her friend were doing. The absence of a verb or verb phrase makes this a fragment as well. Many fragments can be turned into sentences or independent clauses simply by adding a subject or verb. When constructing sentences, include a verb that expresses either an action or state of being.
To determine whether or not what you have written is a sentence or a fragment, ask yourself the following questions: Does my sentence have a verb? Does my sentence have a subject? Does what I wrote make sense on its own? Ask yourself who or what is doing the action and what action is being done. Sometimes this is more challenging when the verb is expressing a state of being or is a helping verb. For example, "The chef's apple pie is the most popular dessert in the restaurant" is a sentence with a linking verb. Sometimes it’s difficult to identify verbs like this because there is no action. Rather, the verb is expressing a state of being. Because sentences like these express complete thoughts, they are not fragments.
One of the best ways to make sure that fragments don’t make their way into your writing is to carefully revise your written work. Review each sentence by locating the verb and the subject of the verb. If either of these elements is missing, you have a sentence fragment. If both a subject and a verb are present, check to see that the group of words is a complete thought. If all three -- verb, subject and complete thought -- are present, you have a sentence.