In the English language, every sentence has two main pieces. The "subject" is the noun of the sentence, and the "predicate" is the verb. Singular subjects take singular verbs, and plural subjects take plural verbs. In other words, the subject and verb must "agree." Of course, English is a much more complex language than just "noun plus verb."
Subject of a Sentence
The subject is a unit of syntax that functions as one of the two primary parts of a basic sentence. It is the person or thing that the sentence is talking about. It is most commonly a noun or noun phrase ("The boy ran"'; "The group of children played"), but it can also be a verb form that functions as a noun ("Hiking is good for one's health"; "To meditate is good for one's soul").
Predicate of a Sentence
The predicate is the other basic unit of sentence structure and can be a little trickier than the subject. It expresses the action (through verbs such as "walk" or "read") or the state of being (through verbs such as "is" or "are") of the subject. The predicate modifies the subject, or helps to describe it further, and carries the tense of the sentence. The predicate must contain a verb, but it can be a verb alone or a verb plus other modifiers.
Basic Types of Predicates
English professors and linguistic experts can categorize predicates into a dizzying number of types, but the basic are a verb alone ("He writes"); a verb with a direct object ("The girl ate a cookie"); a verb with an indirect object ("The audience listened to the music"); and verb with an adjective ("He is handsome"); and a verb with an adverbial phrase ("She is at the store").
A basic rule of English grammar is that the subject and predicate of a sentence must "agree." The subject will govern the agreement, meaning it decides the number (singular or plural), which the predicate must follow, regardless of other elements in the sentence. A simple example of this is: "The boy is thinking" versus "The boys are thinking." As "boy" switches from singular to plural, the predicate must agree by switching from "is" to "are."
Further Rules of Agreement
Most sentences aren't as simple noun + verb. For example, the subject and predicate can be inverted ("In the street hurries the old woman"). Clauses and phrases can sometimes come between the subject and predicate, but they do not change the agreement. ("The governor, who had faced a tough opponent in the last election, wants to begin his campaign earlier this year" can be simplified to "governor ... wants.") If "each" or "every" precedes a compound subject, the subject becomes singular ("Each boy and girl is to go home now").