The Independent Clause and Simple Sentence
A sentence is made up of one or more clauses. To understand what a sentence is, it helps to understand what a clause is. To do that, you must understand what a subject and verb are.
Clause
A clause is a group of words that contains a subject and a verb. A clause may have other words besides these, but to be a clause, it must have the two.
Subject
A subject is usually a noun (person, place, thing, idea) or a pronoun (a word that substitutes for and refers to a noun: e.g. "he" for "Sam"). The subject of a sentence is the noun or pronoun that name who or what the sentence is about:
Fish swim. (Fish is the subject.)
He wrote the letter. (He is the subject.)
Verb
A verb conveys action or "states of being or mind."
Fish swim. (Swim is the verb.)
He seems happy. (Seem is the verb.)
It is not always easy to recognize verbs which convey states of being or mind, but they are some of the most used. Examples are: to be, to seem, to have, to appear.
In English, subjects usually come before verbs:
The battle rages.
However, sometimes the subject comes after the verb:
In our neighbours' yard are two big maple trees.
There are several books on the table.
Subject-verb order is often reversed or modified in questions:
Who are you?
Are you going home this weekend?
In something called an "expletive construction," the subject follows the verb and the sentence begins with either "there is" or "there are."
There is a book on the table.
There are several books on the table.
Independent Clause
An independent clause has a subject and a verb, and by itself, can be a sentence: a simple sentence. All of the above example sentences are independent clauses and simple sentences.