To my ear, the difference lies in whether or not the clause is essential. In this case, it's non-essential (set off by commas) and cannot be omitted. The door was opened by an old woman he had seen before.
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LeeSang"An old woman, whom he had seen before, opened the door.The comma cannot be left out because the relative clause is nonrestrictive or nondefining. Both terms are used. Please use the Search Box to find out more about restrictive and nonrestrictive relative clauses.
AvangiSo tell me, my friend, what's the difference between "essential" and "defining"?I don't think there's any difference. Different terms are used to describe the same thing. I don't think I had ever heard "essential" before I hit EF. That doesn't of course mean that the term is worse than mine.
Avangi CJ used the term "referent" recently. Does that differ in any significant way from an "antecedent?"I don't think so. Relative clauses are simple and straightforward in English grammar compared with the use of participles. I mean that there are few, if any, exceptions and the use of the comma is strictly regulated. Such rigidity isn't always peculiar to