In the examples, subjects are in italics and verbs are in bold.

Sentence

A sentence expresses a complete thought. A simple sentence consists of a subject and verb.

Fish swim.

Subject

The subject names who or what the sentence is about. Some subjects are simple, identifying only one noun or pronoun.

Fish swim.

Other subjects are compound. Compound subjects contain two or more simple subjects joined with a coordinating conjunction such as "and" or "or."

Fish and turtles swim.

Implied Subjects

In imperative (advice or command) sentences, the subject is an understood you, though it may not appear in the sentence.

[You] go fish.

Verb / Predicate

A verb expresses action or state of being.

Fish swim.

Fish are feisty.

Predicate is the grammatical term given to the verb plus its objects, complements, and modifiers.

Fish swim in the Missouri River.

Direct Object

A direct object completes the meaning of the verb. A simple direct object, always a noun or pronoun, answers “what” or “whom.”

The fish gave kisses.
The fish gave what? The fish gave kisses (direct object).

Indirect Object

The indirect object of the verb comes before the direct object and usually tells to whom or for whom the action of the verb is done.

The fish gave their offspring kisses.
The fish gave kisses to whom? The fish gave kisses to their offspring (indirect object).

Phrases

Phrases are word groups that function as adjectives, adverbs or nouns. They can’t stand alone because they don’t contain both a subject and active verb. Below are some examples:

Fish swim in the Missouri River.
This prepositional phrase functions as an adverb because it describes or modifies swim.

Fish, our new pets, swim in the Missouri River.
The appositive our new pets renames the subject fish. Appositives are noun phrases that rename nouns or pronouns.

To fish is fun.
An infinitive phrase consists of [to] followed by a verb [to fish]. This infinitive is a noun, the subject of the sentence.

There Are / There is

Verbs ordinarily follow subjects. But this normal order is reversed when a sentence begins with there is or there are. In these cases, the subject follows the verb.

There are too many fish in the Missouri River.
The subject fish is plural, so it takes a plural verb.

There is one fish in the Missouri River.
The subject fish is singular, so it takes a singular verb.