Learn English – Omission of “when” in relative clauses

relative-pronouns

One of my students had to fill in the gap (or leave it blank if the relative pronoun can be omitted) in this sentence:

My landlady says we can't use her sitting-room because we made too
much mess the last time ________ she let us have a party.

He wrote "which". The answer key left it blank.

"Which" is not an option, while a blank or "that" or "when" are possible options. But I am having a hard time explaining to him why "when" can be omitted since grammar books seem to claim you can only leave out the relative pronouns "who/which/that".

Please correct me if you find any statements above are wrong.

And my question is: what makes it possible for "when" to be omitted? If possible, quote source for information.

Best Answer

We can generally omit relative pronouns from defining relative clauses if the wh- word does not represent the Subject. The word when replaces temporal Adjuncts, so if the clause is a defining relative clause we can nearly always remove it.

  • That was the day when you asked me to marry you.
  • That was the day you asked me to marry you.

One case where this does not seem to apply is with relative clauses using the relative word where, although for some strange reason it seems to be marginally acceptable when the antecedent is the word place:

  • That's the restaurant where we met last time.
  • *That's the restaurant we met last time. (ungrammatical)
  • That's the same place we met last time.
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