A Clause is nothing but a group of words which at times might form part of a sentence or might be a complete sentence in itself. It consists of a subject and a verb and is considered to be the smallest grammatical unit which makes sense. They can be mainly classified under two heads based on their meaning:
1. Independent or Main Clause (one which expresses complete meaning and can stand on its own)
2. Dependent or Subordinate Clause (one which cannot make a complete meaning on its own and cannot stand alone and need the help of an Independent Clause to form a sentence) which can be further divided under three heads:
a) Noun Clause (A dependent Clause which acts as a noun)
b) Adjective Clause (A dependent Clause which acts as an adjective)
c) Adverbial Clause (A dependent Clause which acts as an adverb)
A Dependent Clause (who, whom, whose, that, which etc.), acting as an adjective is known as an Adjective Clause. Also known as Relative Clause functions similar to an adjective i.e. it also modifies a noun or pronoun. It will satisfy the below conditions, i.e.
It acts as an adjective and thus answers the questions like which, what, whom, who, etc.,
It generally begins with a relative pronoun (that, who, which, whose, whom) or a relative adverb (when, why, where).
There are two types of Adjective Clause:
Restrictive or Essential or Definite Clause
Non Restrictive or Non-Essential or Non Definite Clause
Non Restrictive Clauses:Non Restrictive Clause too is just the opposite of the Restrictive Clause. It does not restrict or limit the noun it modifies and is set off by commas unlike the Restrictive Clause. It is just gives us additional information which is not so essential to the sentence, and thus even removing or deleting the Non Restrictive Clause will not cause much difference to the sentence.
I went to the museum with Sam, who lives next door.
My sister, who is an excellent dancer, is moving to London.
In both the above sentence the clauses even if removed will not create much difference to the sentence.
A non-restrictive clause is that which conveys some insignificant or unnecessary information about the antecedent in the main clause, which is not essential for the sentence to make sense, and so can be omitted without changing the meaning of the sentence. This is the reason why relative pronouns that introduce a non-restrictive clause are commonly separated from the main clause by a comma.