Sometimes the relative pronoun or the relative adverb introducing an adjective clause is understood and is hence omitted.
- This is all I have. (= This is all that/which I have.)
- Here is the camera I promised to give you. (= Here is the camera which / that I promised to give you.)
- The reason she hates me is unknown to me. (= The reason why she hates me is unknown to me.)
An adjective clause may be defining or non-defining.
A defining adjective clause clearly identifies its antecedent whereas a non-defining adjective clause merely gives some information. In writing, non-defining adjective clauses are always separated by commas.
Examples of defining relative clauses are:
- There are the keys that you were looking for.
- This is the house that Jack built.
Examples of non-defining relative clauses are:
- Susan, who is a well-known social activist, is a brave woman.
Here the adjective clause ‘who is a well-known social activist’ merely gives some additional information about Susan. Hence it is a non-defining adjective clause.
- Jack, who is my friend, lives abroad.
Here the adjective clause ‘who is my friend’ is non-defining because it merely adds some additional information.
The relative pronoun can sometimes be omitted in the case of a defining relative clause.
- The book you gave me was very interesting. OR The book which you gave me was very interesting.
The relative pronoun introducing a non-defining adjective clause cannot be left out.
- Susie, who is my sister, is a doctor. (NOT Susie is my sister is a doctor.)
Note that a non-defining adjective clause can be left out from the sentence without altering its meaning.