Short answer and quick fix:
Look at the gap in the relative clause. If the gap can be filled in with the pronoun it, use the relative pronoun which. If the gap can be filled in using the locative preposition there, use the relative word where:
- That's the restaurant which [I hate ____ ].
- That's the restaurant which [I hate
it].
- That's the restaurant where [I met my wife ____ ].
- That's the restaurant where [I met my wife
there].
Note that the strikethough across the words it and there indicate that we cannot actually leave these gaps filled!
Full answer:
This question is about relative clauses. These are special clauses with gaps in them. They modify other phrases, in particular noun phrases, which is the kind of case we are considering here.
The nominal or noun phrase which is being modified always occurs before the relative clause, and is referred to as the ᴀɴᴛᴇᴄᴇᴅᴇɴᴛ. So in the dog, which they adopted, the phrase the dog is the antecedent, and the phrase which they adopted is the ʀᴇʟᴀᴛɪᴠᴇ ᴄʟᴀᴜsᴇ modifying that noun phrase. Relative clauses are often introduced by wh-words such as which, who or where, or by the relative word that.
The word which is a pronoun and can be thought of as standing in for noun phrases. The word where—according to 21st Century grammars such as The Cambridge Grammar of the English Language (Huddleston & Pullum 2002) or Oxford Modern English Grammar (Aarts 2011)—is a locative preposition and can be thought of as standing in for preposition phrases (some people conjecture that where is a 'pro-preposition').
The Question
It is very confusing for students and teachers alike when considering why we use which or where when the antecedent of a relative clause can be thought of as a place:
- That's the restaurant where I met my wife.
- That's the restaurant which I hate.
I've heard teachers and linguists try to explain this in terms of phrases like the restaurant representing a place in the first example and a thing in the second. This is just silly.
The truth is that whether we use which or where in the sentences above doesn't depend on the status of the antecedent, the restaurant, either conceptually or in terms of its role within the larger sentence. Rather, it depends on the grammatical function that that phrase would occupy in the relative clause itself.
If we look at the sentences above , we can see that there is a gap in the clauses following the relative wh-word:
- That's the restaurant where [I met my wife
at the restaurant]
- That's the restaurant which [I hate
the restaurant]
We can see that the relative word is co-indexed with that gap, so that we understand the relative word to refer to the missing portion of the clause:
- That's the restaurant where(i) [I met my wife ____ (i)]
- That's the restaurant which(i) [I hate ____ (i)]
Now, you'll notice that the gap in the relative clause in the first sentence can be replaced with a preposition phrase and the gap in the second sentence can be replaced by a noun phrase. As a rough rule of thumb, we can say that when the relative word is indexed with a gap fillable with a noun phrase, we use which (or that), and when it is indexed with a gap fillable with a preposition phrase, we use where. That is why the it/there test described in the short answer works. It is a pronoun which can work as a noun phrase. There is a locative preposition which can stand in for a larger preposition phrase.
However, it is probably more accurate to consider the grammatical relations that the gap has within the relative clause (rather than what type of phrase it could be replaced with) as being the crucial factor. So it's best to think in terms of whether the missing phrase is a Subject, Object, Predicative Complement or Object of a preposition—in which case we need to use which— or whether it is a Locative Adjunct, or the Locative Complement of a verb—in which case we need to use where.
To see why this might be a better description consider the following:
- She ate using her fingers, which I hate ___. (Object)
- She ate using her fingers, which ___ annoyed me. (Subject)
- She ate with her fingers, which everyone was horrified at ____. (Object of a preposition)
Here the word which can be thought of as representing a finite clause, instead of a phrase headed by a noun. Certainly in the first two sentences the gap in the relative clause could be replaced with one (if it was a stand-alone sentence):
- I hate [that she ate with her fingers].
- [That she ate with her fingers] annoyed me.
Of course, those gaps could also be plugged with a pronoun:
- I hate it.
- It annoyed me.
In the third example, the word which seems to be referring back to a clause, but the gap in the relative clause can only really be thought of as representing a noun phrase:
- Everyone was horrified at it.
- *Everyone was horrified at that she ate with her fingers.
Nonetheless, it is hard to show for sure that in the first two examples, the gap represents a pronoun or other noun phrase, rather than a clause. There are also instances where we might consider the relative relative word which and the gap it is indexed with to be representing an adjective phrase or verb phrase, for example. It may be simpler and more consistent, therefore, to refer to the grammatical function of the gap within the clause, rather than the type of phrase which is being deleted.
Here are some examples where the gap is functioning as a Locative Complement or Locative Adjunct:
- This is the drawer where [I put my pen ___ ]. (Locative Complement)
- That is the town where [I used to live ___ ]. (Locative Complement)
- That is the park where [Bertha plays football ___ ]. (Locative Adjunct)
- That is the pub where [I bet Bob £5 that Scotland would beat England in the rugby _____ ]. (Locative Adjunct)
Notice that a sense of location is not enough to make us want to use where. If we leave a normal preposition in place in the third example above, the gap will become the Object of a preposition, in which case we will need to use which, not where - even though the park obviously represents a location:
- That is the park which [Bertha plays football in ____ ].
Grammar notes:
Many traditional grammars regard where and there as adverbs instead of prepositions. This is fine and doesn't affect the story above very much - apart from that we lose the neat general correspondence of prepositions usually representing preposition phrases and pronouns usually representing noun phrases within relative clauses.
Adjuncts are embellishments that we stick onto well formed clauses to give extra information. They're often preposition phrases or adverbs. Complements, in contrast, fill a special slot set up by the verb. So in the park is an Adjunct in She plays football in the park but a Complement in We put the statue in the park (consider we put the statue where there seems to be some kind of missing information). Locative Adjuncts or Complements are merely ones that tell us about goals, sources or locations. They have their own special behaviours.
Best Answer
There are four factors which decide whether a relative pronoun (or the word that) can be omitted or not:
If the answer to the questions above is yes then the relative pronoun can be omitted. There are lots of duff websites around which might tell you that you can only omit these words if they represent the Object of the relative clause. This is hogwash. You can omit the pronoun as long as it is not the Subject of the matrix verb in the relative clause. The pronoun can, for example, be omitted if it is the Object of a preposition or Subject of another clause embedded within the relative clause.
Here are some examples to illustrate each point. An asterisk, *, denotes an ungrammatical example.
We cannot usually drop a pronoun from a non-defining relative clause:
We cannot drop the relative pronoun if the main verb in the relative clause does not have a separate Subject:
Notice that in the second example the missing pronoun represents the Subject of the verb bit.
If the pronoun is embedded within another phrase, for example a preposition phrase, then it cannot be omitted:
We can drop the pronouns who, which and the word that, but we cannot drop the pronoun whose:
The Original Poster's example
The word someone here has been post-modified by an adjective phrase. Some people argue that this is the result of removing who is from a relative clause. If you have a defining relative clause which uses the verb BE, you can often drop the relative pronoun and the verb BE. Whether it is now a kind of relative clause, or just an adjective phrase, or participle phrase modifying the noun is up for debate. Here are some more examples:
Here's an example where you can't:
We often cannot do this if what's left of the clause is only one adjective.
The sentence has a real relative clause modifying the phrase tiny creatures:
This is a defining relative clause, and the verb get has its own Subject, the word we. There are no other words preceding which. We can therefore happily drop the relative pronoun.
We could rewrite the sentence like this to show where potential words have been omitted:
Note:
Many grammars use the terms restrictive/non-restrictive or integrated/supplementary to describe what I've called defining and non-defining relative clauses.