English is a beautiful and challenging language. Even those of us who grew up in English-speaking homes sometimes struggle with the finer points and have a hard time recollecting exactly what they were taught about how to diagram a sentence. Predicate nouns aren't hard to grasp when explained; like many grammatical concepts, this one sounds more complicated than it actually is.
Basic Sentence Structure
A simple sentence has two or three components. "Mary ran," is a two-part sentence consisting of a noun, or subject (Mary), and a verb, or predicate (ran). "Mary ran home" is a three-part sentence with a subject, a predicate, and an object (home).
Types of Predicates
All verbs are predicates, but all predicates are not just verbs. The World English Dictionary defines "predicate" as "the part of a sentence in which something is asserted or denied of the subject of a sentence; one of the two major components of a sentence, the other being the subject."
Nouns and Pronouns
A noun is a person, place or thing. Proper nouns are specific to a particular person or place (Mary, New York) while other nouns are not (girl, city). A pronoun such as he, she or they takes the place of a proper noun.
The subject of a sentence is always a noun or pronoun.
Predicate Nouns and Pronouns
A predicate noun or pronoun is part of the predicate of a sentence rather than being the subject and serves to modify or describe that subject. The predicate noun tells what the subject is. "Mary is president." "Fred is a registered nurse." In the last example, nurse is the predicate noun, while registered is a predicate adjective that describes what kind of nurse Fred is. A predicate pronoun replaces a predicate noun. "Mary gave it to me."
Predicate nouns and pronouns can also be called predicate nominatives.