What is clause?
A clause is a group of words that has a subject and a verb.

Kind of clause.

1. Independent clause
    An independent clause can stand alone as a simple sentence.

    My name is Mr. Lee.

2. subordinate clause
    A subordinate clause has a subject and a verb but can not stand alone as a sentence. Subordinate clauses are introduced by subordinating conjunctions such as since, because, when, if, and while.

    If she read Watership Down, she would like it.

3. adjective clause
    An adjective clause is a subordinate clause that modifies a noun or pronoun. Adjective clauses are often introduced by relative pronouns.

A flower that I like  is rose.

4. essential clause
    An essential clause is an adjective clause that is necessary to the meaning of the sentence, for without it the sentence would have a different meaning. Essential clauses are not set off with commas.

    The character whom the novel is about is a prince.

5. Nonessential clause
    A nonessential clause is an adjective clause that is not necessary to the meaning of the sentence but merely adds information. Use commas to set off nonessential clauses.

    The little prince, who is the main character in the book is from another planet.

6. adverb clause

    An adverb clause is a subordinate is a subordinate clause used as an adverb.

    Motzart wrote many compositions before he was fourteen.

7. noun clause
    A noun clause is a subordinate clause used as a noun. Noun clauses can be used in the same ways a nouns in sentences.

    Whoever wrote that sonata was talented. (subject)
    I admire whatever she does. (direct object)