1. When the relative pronoun is the object of the relative clause:
* Example: "The book I bought yesterday is very interesting." (Here, "I bought" is the relative clause, and "I" is the object. We can omit the relative pronoun "which" or "that".)
2. When the relative pronoun is the object of a preposition:
* Example: "This is the house I live in. " (The preposition "in" is followed by the relative pronoun "which" or "that," which is the object of the preposition. We can omit the pronoun.)
3. In informal writing and speech:
* Example: "The person helped me is my friend." (In informal contexts, you might omit the relative pronoun even if it's the subject of the relative clause.)
However, it's important to note:
* Formal writing generally requires the relative pronoun, even when it's the object. So, it's safer to include the relative pronoun in formal writing.
* Clarity is paramount. If omitting the relative pronoun creates ambiguity, it's best to keep it in. For example, "The person helped me is my friend" could be misinterpreted without a relative pronoun.
Examples with relative pronouns:
* Formal: The book which I bought yesterday is very interesting.
* Formal: This is the house in which I live.
* Informal: The person that helped me is my friend.
Examples without relative pronouns:
* Informal: The book I bought yesterday is very interesting.
* Informal: This is the house I live in.
Remember, the decision of whether or not to omit a relative pronoun is a matter of style and context. Consider the formality of the situation and the potential for ambiguity before making your choice.