Only two hard-and-fast requirements exist for grammatical sentences: they must have a subject -- a noun or noun phrase that performs an action -- and a verb, or action word. That means a sentence can be as short as "Joey ran," where "Joey" is the subject and "ran" is the verb. Once modifiers are introduced to sentences, though, grammar becomes less simple. Once of the stickier points is telling a direct object from a predicate nominative.
Two Kinds of Verbs
A key difference between direct objects and predicate nominatives lies in what type of verb they follow. Predicate nominatives come after linking verbs. Linking verbs usually describe a state of being: any form of "to be," such as "is," "are" or "were," and verbs like "grow," "continue," "become" and "turn" are linking verbs. Sensory verbs such as "look," "sound" and "feel" are also linking verbs. Direct objects come after transitive verbs, which describe an action that the subject takes, such as "predict," "throw," "answer" and "open."
For a word or phrase to be a predicate nominative, it has to be a noun that defines the subject and it has to follow a linking verb. To test for predicate nominative status, try inserting the linking verb "is" between the subject and the word or phrase in question. For example, in the sentence "Henry Tudor became the king," "the king" is a noun phrase that follows a linking verb, and you could say, "Henry Tudor is the king," so "the king" is a predicate nominative. But in the sentence "Mary appeared shy," "shy" is an adjective, not a noun, although you could say, "Mary is shy." Instead of being a predicate nominative, then, "shy" is a predicate adjective.
For a word or phrase to be a direct object, it has to be a noun that receives a transitive verb's action. Usually that means that the subject of the sentence is doing something to it. For example, to determine the direct object in the sentence "That dog ate my chicken burrito," first find the main verb: "ate." Next, ask what noun "ate" acts on -- that is, what did that dog eat? "My burrito" is receiving the action of the verb "ate," so it's the direct object of the sentence. Since "my burrito" doesn't define "that dog," and "ate" isn't a linking verb, "my burrito" can't be a predicate nominative.
Another noun called the "objective complement" can accompany the direct object, and it's sometimes confused with either the predicate nominative or the direct object. Objective complements are nouns that come after a transitive verb, as direct objects do. Instead of defining the subject, as predicate nominatives do, objective complements define the direct object. In the sentence "The students elected Gina class president," "Gina" is the direct object, since she receives the action of being elected. The noun "president" defines "Gina," not "the students," so it's an objective complement.