In the sentence "A heavy book fell off the shelf," the book is the subject, and "fell off the shelf" is the predicate. In the imperative sentence "Take the dog for a walk," the entire sentence is the predicate. The subject is "you," which is implied because the speaker is directing the command at another person.
The subject is the word or phrase that defines what the sentence is about, while the predicate describes the action the subject is performing or the subject's state of being. A grammatically correct sentence must have both elements.
In the most basic sentences, the subject and predicate may only be one or two words, as in the clause "Mary is happy." Mary is the subject, and "is happy" is the predicate. However, modifiers and phrases are often necessary to enrich the detail of a subject or predicate. In the opening example, "A heavy book" is the complete subject. The adjective "heavy" modifies the simple subject "book" to distinguish it from a small or lightweight book.
Similarly, the simple predicate is the core verb, while the complete predicate includes any accompanying parts of speech necessary to convey the full action or condition. In the book example, the word "fell" is the simple predicate, and the prepositional phrase "off the shelf" answers the question of where the subject performed the action.