Adjective Clauses

This page is about adjective clauses and relative clauses.

Definitions:
Relative [=adjective] clauses
Necessary [=defining; restrictive] clauses
Unnecessary [=non-defining]

There are two different points which should not be confused:

1. Omitting just the relative pronoun if possible
2. Reduction of relative clauses if possible. In these cases the relative pronoun will be omitted and there may be some changes to the verb within the relative clause.

Concerning the first point, “The relative pronoun can be omitted ONLY in necessary [defining] adjective [relative] clauses.”

What is called “reduction” is something else. It is not limited to only defining relative clauses. There are conditions for reducing relative clauses, whether defining or non-defining.

Pollock (1997:149-150) explains as follows:
“An adjective clause can be reduced to an adjective phrase only if the clause (a) begins with who, which, or that as the subject of the clause and (b) contains a be form of the verb. … If there is no be form of the verb, it is often possible to omit the relative pronoun and change the verb to its –ing form [present participle].”

It is not limited to only defining clauses. On the section about reduction of full adjective clauses into adjective phrases, the  grammar book goes on with this clarifying point, with a warning about preserving the original punctuation:
“If the adjective clause needs commas [i.e. non-defining], the adjective phrase also needs commas.”

Examples:

“Adjective clause: You can get your passport renewed at the Kennedy Building, which is located near the train station.
Adjective phrase: You can get your passport renewed at the Kennedy Building, located near the train station.”

Adjective clause: These articles, which were written several years ago, have been published in several popular magazines.
Adjective phrase: These articles, written several years ago, have been published in several popular magazines.

The examples above are non-defining clauses which have acceptable reduced forms.

Appositive Phrases

A related kind but with different name is appositive phrases:

'Some adjective clauses can be reduced to appositive phrases. An appositive phrase is a noun or pronoun with modifiers that is placed after another noun or pronoun to explain it.'

Adjective clause: History, which is my favorite subject, has always fascinated me.
Appositive phrase: History, my favorite subject, has always fascinated me.

Because an appositive phrase adds only extra, unnecessary information, it is always set off from the rest of the sentence by commas.'

Appositive phrases are instances of non-defining relative clauses which are reduced with the same sets of rules.

(Source: Communicate What You Mean: A Concise Advanced Grammar, 2nd Edition, by Carroll Washington Pollock Longman, 1997, pp. 149-150).

(Thanks to Piruz Siavoshi for this information).