Student or Learner
Can you say "This is the place we used to play." without "where" inbetween"? In 2, "when" can be omitted. If "where" can't be omitted, there seems to be no consistent rule for this.
1. This is (the place) where we used to play.
2. We've come to the time (when(that)) we have to make a decision.
In your first sentence, "where" can be omitted. The problem with the second sentence is that "have" can have different meanings. If you omit the relative pronoun there, the reader might stumble on "have", reading that "the time we have" is the time we possess. It is clearer with the relative pronoun "when".
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