1. When it's grammatically correct:
* Separate clauses: If the relative pronouns introduce separate relative clauses that modify different nouns, they are perfectly acceptable:
* "The book that I bought which is about history is very interesting." Here, "that" modifies "book" and "which" modifies "book" (referring to the specific book being discussed).
* Nested clauses: A relative pronoun can introduce a clause that modifies another relative pronoun, creating a "nested" structure:
* "The man who lives next door whose dog barks all night is very annoying." Here, "who" modifies "man," and "whose" modifies "man" (referring to the man specifically).
2. When it can be awkward:
* Redundancy: If the relative pronouns refer to the same noun and have the same function, it can be redundant:
* "The dog that I saw that was chasing a squirrel was very cute." (Here, both "that" clauses modify "dog" in the same way. It's more concise to say "The dog that I saw chasing a squirrel was very cute.")
* Clarity: Too many relative pronouns can make the sentence difficult to follow. Sometimes, using a different structure can improve clarity.
3. The "that" rule:
* In formal writing, "that" is typically preferred over "which" when the relative pronoun is essential to the meaning of the sentence. However, "which" can be used if the clause is non-essential.
Example:
* "The book that I bought is interesting." (Essential clause – "that" tells us which book we're talking about)
* "The book, which I bought yesterday, is interesting." (Non-essential clause – "which" adds extra information)
In summary:
Using two relative pronouns in the same sentence is sometimes necessary and grammatically correct. However, be mindful of redundancy and clarity to avoid creating awkward sentences.