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.
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.
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