* Predicate nouns are nouns that re-name or describe the subject of a sentence. They follow a linking verb (like "is," "are," "was," "were," "seems," "becomes").
* Prepositional phrases are groups of words that start with a preposition and modify another word in the sentence. They tell us where, when, how, or with what.
Example:
* The book on the table is a mystery.
* "Book" is the subject.
* "is" is the linking verb.
* "a mystery" is the predicate noun, re-naming the subject.
* "on the table" is a prepositional phrase telling us the book's location.
Think of it this way: A predicate noun must be a noun (or pronoun) that acts as a second subject, while a prepositional phrase is a group of words that functions differently.
Let me know if you have any other questions!