Is "where" the only relative pronoun that cannot be omitted from an adjective clause?

When using adjective clauses, the relative pronoun can be omitted when it is not the subject of the sentence. For example:

"She is the person I ran into."

In the above example, being the object of the sentence, the relative pronoun "who" (or whom) was left out.

BUT

This is true as long as the relative pronoun is not "where". In case of "where", even if it's not the subject of the sentence, it cannot be omitted from the sentence. It can only be replaced with "that" or "which", and that requires a preposition at the end of the sentence. For example:

"This is the building we met." is wrong, but

"This is the building where we met." is correct, as is "This is the building that/which we met in."

You don't have to do this with other relative pronouns. They can be left out from the sentence without a problem, as in "I'm thinking about the time we met." The pronoun "Where" is the only one that cannot be omitted from the clause altogether.

I just need to know if I'm right about this. Thank you.

Solution 1:

Non-pronoun Wh-relative words have idiosyncratic restrictions, and enter into many idiosyncratic constructions. Where is a good example; so is why.

Both where and why are less common than that in relative clauses relating location or purpose, but I'm going to ignore that here; it's a separate issue, and only arguably a relative pronoun in any case.

Where can be deleted from its relative clause (without substituting that), but only in a relative clause whose antecedent is the word place. Similarly, why can be deleted only when its antecedent is the word reason. Other words, even words denoting places or reasons, just don't work with these Wh-relative words.

• That's the place (where/that) we met/lived.
• *That's the location/house/city/apartment/block we met/lived
• That's the reason (why/that) they came/argued.
• *That's the purpose/cause they came/argued

This is either the result of or the cause of the non-constituent fixed phrases place where and reason why. Other similar phrases don't seem to be tidelocked into one particular noun, though.

• That's the time (when/that) we quit/he wants to leave.
• That's the hour/day we quit/he wants to leave.

Of course, these restrictions pale beyond the restriction on how -- it can't be used as a relative pronoun at all, no matter what its antecedent is. Either that or zero is used instead.

• *He doesn't know the way how he should salute
• He doesn't know the way that he should salute.
• He doesn't know the way he should salute.

Like what -- which also can't be used as a relative pronoun -- how can be used to mark a headless relative, where what equals 'that which' and how equals 'the way which'.

• He doesn't know what he should do.
• He doesn't know how he should salute.

These are also common with relative infinitives, which include modal senses like should.

• He doesn't know what to do.
• He doesn't know how to salute.

But relative infinitives are way too complicated to discuss here, thank gods.

Solution 2:

You're confusing relative pronouns and relative adverbs here.

Who/Whom, That/Which are indeed relative pronouns, but Where is a relative adverb.

I believe the confusion stems from the fact that both relative pronouns and relative adverbs refer back to an antecedent. While that's true, that's not the whole story.

Relative pronouns and Relative adverbs function differently in a sentence: the former as objects and the latter as adjuncts.

Solution 3:

Although you can think of "She is the person I ran into" as involving omission of the relative pronoun who(m), I think the clearest way to describe relative clauses with no explicit relative words is by comparing them to relative clauses starting with that. If there is no corresponding that relative clause, you won't be able to leave out a relative word:

• No *"This is the building that we met" (it is syntactically possible, but doesn't make sense), so also no *"This is the building we met."

In general, that can be omitted from a relative clause that contains a subject:

• "She is the person that I ran into" > "She is the person I ran into"

That clauses are most often interchangeable with clauses containing the relative pronouns which or who(m). However, that can also be interchangeable with the relative words where or when in relative clauses coming after certain special types of noun phrases:

• "I remember the day when I moved in to my apartment"
  "I remember the day that I moved in to my apartment"
  "I remember the day I moved in to my apartment" (could be seen as "omission of when")

A noun phrase headed by the noun place can marginally, for some speakers, allow a that relative clause or a clause without an explicit relative word:

• "This is the place where we met"
  ?"This is the place that we met"
  ?"This is the place we met"

See my answer here for more details and references: Relative pronouns “where” and “when”: where can they be omitted?

Both where and when are often not categorized as pronouns when they are used to form relative clauses. Their categorization is argued (as you can see, Araucaria's answer to the linked question says that where is a locative preposition or "pro-preposition"; traditional grammar calls it an "adverb" in this context.)