We have already learned that a relative pronoun can act as the object in a relative clause. When a relative pronoun acts as an object, it may be omitted. This omission of the relative pronoun is more common in an informal style.

• The woman whom you were talking about is my boss. (Formal)
• The woman you were talking about is my boss. (Informal)

Here the relative pronoun whom is the object of the relative clause you were talking about.

When the relative pronoun is the object of a preposition, which is used instead of that.

Study the example given below.

• We bought some apples. We extracted the juice from them.

Here the pronoun them is the object of the preposition from. When we combine these two clauses, we use the relative pronoun which instead of that.

• We bought some apples from which we extracted the juice. (NOT We bought some apples from that we extracted the juice.)
• She bought me a beautiful necklace. I was very grateful for that.
• She bought me a beautiful necklace for which I was very grateful. (NOT She bought me a beautiful necklace for that I was very grateful.)
• He lent me his mattress. I slept soundly on it.
• He lent me his mattress on which I slept soundly.
• There was a tow hitch on the car. We attached the trailer to it.
• There was a tow hitch on the car to which we attached the trailer.