The use of this presupposes discursive or actual proximity to the referent.
Question: "I read this great novel about 30 years ago.
Another user objected that the use of the word "this" was incorrect in this context. He wrote:
I do not think it is correct to use "this" when writing about something [as yet] unidentified." [bolding mine]
I agree, the indefinite determiner a would be used as it has not yet been identified:
I read a great novel about 30 years ago. It was about [whatever].
BUT it can also be:
- I read this great novel about 30 years ago. It was about aliens and cowboys on ship in the Pacific.
That use of "this" refers to the speaker's actual experience, which is then revealed in the next statement. This question is really about deixis:
Common Frame of Reference Needed Without a common frame of reference between the speakers, the deixis on its own would be too
vague to be understood, as illustrated in this example from Edward
Finegan in "Language: Its Structure and Use."
Consider the following sentence addressed to a waiter by a restaurant
customer while pointing to items on a menu: "I want this dish, this
dish, and this dish". To interpret this utterance, the waiter must have
information about who I refers to, about the time at which the
utterance is produced, and about what the three noun phrases this dish
refer to." (5th ed. Thomson, 2008)
See example
this can refer to something mentioned a speaker to references in his/her own speech but usually it does not refer to something in someone else speech. If two speakers are addressing each other, one might might use this if the context is very clear.
Speaker 1 (quoted in a slide): "We had a huge problem with our tap
water. It turned out to contain much too much lead. We were afraid our
kids would be poisoned".
Speaker 2 (speaking to Speaker 1 or
addressing a roomful of students, for example). Yes, we have found
this problem to be very serious in certain towns using the old water mains.
Summary: this as an actual thing (a referent) the speaker and listener are in contact with (the waiter example above) or this as a catch-all pronoun that refers to something in a speaker's own discourse or in close proximity to the speaker and listener(s) (the slide example). Both of these uses are deictic as they "point to the thing", actual thing or thing in speech (topic, situation).