1. Reduced clauses are a common feature of written English. They are not much used in spoken language.
2. As their name implies, they are created by 'reducing ' or contracting a standard clause.
3. They are a stylistic device. In other words, they are used for style, not for any other reason.
REDUCED ADJECTIVE CLAUSES
1. Teachers who work here come from many different countries.
Teachers working here come from many different countries.
2.Oil which is produced here is exported elsewhere.
Oil produced here is exported elsewhere.
In example 1, who work becomes working. Why?
Because work is Active and an Active voice is replaced by the Present Participle (V-ing) in a reduced clause.
It doesn't matter what tense the verb is, this rule applies. Look at example 3.
3. Students who have lost their books have to replace them.
Students losing their books have to replace them.
In example 2, which is produced becomes produced. Why?
Because is produced is Passive and this is reduced to the Past Participle in a reduced clause.
You may delete the relative pronoun and the be verb when:
1. they are followed by a prepositional phrase.
A. The man who is in the house is my father.
The man in the house is my father.
B. The books that are on the desk are mine
The books on the desk are mine.
2. the main verb in the relative clause is progressive.
A. The man who is swimming in the lake is my father.
The man swimming in the lake is my father.
B. The books that are lying on the floor are mine.
The books lying on the floor are mine.
You may NOT delete the relative pronoun and the be verb when:
1. They are followed by an adjective:
The man who...