I posted a question quite long time ago asking whether "that was a day that my dog died" is valid. The answers said "no!", telling that a dog cannot die twice.
I thought it was possible, for I thought I was introducing an item for the fist time, for which the indefinite article is necessary.
So I searched around and found this sentence: "this is a book I told you about yesterday."
The answerer was saying that "it might suggest that it was one of several books, or it might mean that you don't think I remember it", which is similar to introducing an item for the first time, because the listener does not have any information about it due to inability to remember it.
But some others said that "this is a book I told you about yesterday can only be sensibly used when the book is one of several that I told you about. Whether I think you remember or not is irrelevant."
So I came to this conclusion:
When a noun modified by a relative clause is used, the indefinite article is used only for the situations in which the noun is one of several.
I was naively happy with the satisfaction that I cleared up some confusion. Yet it occurred again, when I found that:
A woman who fell 10 meters from High Peak was lifted to safety by a helicopter.
is actually valid, even though there was only one woman who fell 10 meter from High Peak (it would be strange if there were more than one).
And more surprising was that the reason for its validity was because it was introduced for the first time, news they said. "This is news. We haven't heard about this woman before."
So I just cannot see the difference between "this is a book I told you about yesterday" and "a woman who fell 10 meters from High Peak was lifted to safety by a helicopter."
Why can a definite article be used with "woman" for introducing for the first time and not with "book"?
Also, these two uses puzzled me even further.
“Everywhere there is growing interest in the Exposition,” he told Burnham in a June 20 letter from Biltmore.
It should be "the" according to the explanation people have given me, right? Unless there is more than one letter he sent to Burnham on June 20, which is unlikely.
While aboard the train, Burnham wrote a letter to Olmsted that contained a less-than-candid description of the meeting with the architects.
Likewise, it should be "the" unless Burnham wrote several letters that contained a description of the meeting.
I fail to notice how this "letter" is different from "day". There was only one letter as there was only one day that my dog died. So why indefinite article cannot be used with "day" as used above?
It became a rather long question.
However, my central question is, what controls the use of the definite and indefinite article when modifying a noun with relative clause?