Subjects and Verbs
To find sentence structure errors, you must be able to identify subjects and verbs. Verbs present the action in the sentence, such as "walked" or "sing." Verbs may also represent a state of being: "am," "is," "are," "was" and "were" make up the primary linking verbs. The subject performs the action or is in the state. For instance, "the boy walked to school" contains the action verb "walked." The person who did the walking is "boy," the subject. Complete sentences contain both subjects and verbs.
Fragments, or incomplete sentences, may be missing the subject, as in "walked to school," which does not explain who walked. Writers sometimes omit the verb instead, like in "the boy in the blue sweater," which is missing an action. Words ending in -ing by themselves do not function as complete verbs, so "the boy walking" also represents a fragment. Groups of words that begin with subordinate conjunctions such as "although" and "since" cannot stand alone, either. For instance, "since the boy walked to school" needs a complete sentence attached to it to be correct.
Run-on sentences, also called fused sentences, occur when writers put two complete ideas together with no punctuation or conjunctions between them. For example, "the boy walked to school his sister went with him" creates a run-on. Looking for the subjects and verbs helps writers determine when run-ons occur. In this case, "boy walked" and "sister went" take the roles of subjects and verbs. Next, examine how the two connect. If no semicolon or coordinate conjunction -- for, and, nor, but, or, yet, so -- appears between the sentences, you have created a run-on.
Writers sometimes attempt to correct run-on sentences by adding a comma between them. However, this addition creates another kind of sentence structure error called a comma splice, as in "the boy walked to school, his sister went with him." Again, look for subjects and verbs first, and then determine how complete sentences connect. As with run-ons, comma splices require a semicolon instead of the comma or a coordinate conjunction right after the comma to be correct.