What part of speech is “this” in the following sentence [duplicate]

My friends and I are having a stupid argument about this sentence:

• Someday this pain will be useful to you.

I claim “this” in this sentence is an adjective modifying pain, but they claim it is a demonstrative pronoun in this instance. Can someone please confirm what part of speech it is? This does not seem hard to me but they seem so confident against it.

Solution 1:

Beyond being a demonstrative pronoun or a demonstrative adjective: how we refer to ideas and situations in English is not always about physical objects.

A deictic expression or deixis is a word or phrase (such as this, that, these, those, now, then, here) that points to the time, place, or situation in which a speaker is speaking. Deixis is expressed in English by way of personal pronouns, demonstratives, adverbs, and tense. The term's etymology comes from the Greek, meaning "pointing" or "show," and it's pronounced "DIKE-tik."

https://www.thoughtco.com/deictic-expression-deixis-1690428 deixis

Sample sentence: Someday this pain will be useful to you.

"this pain" refers to the pain one person hears about from another, when they are talking to each other. Not "that pain" which was referred to at another point in time by the other speaker and which this speaker is now referring to.

Often, it is difficult for non-native speakers to grasp this usage of this/that in a discourse situation.

This is a difficult question. [pronoun]
This question is difficult. [adjective]
That one is easy. [adjective]
That is difficult. [something one person is referring to in another person's speech, pronoun]
This is difficult. [something being referred to by a speaker as "appropriated" by them, pronoun]

Clearly, the word this or that + a noun function as a adjectives AKA determiners whereas this or that without another noun function as stand-alone nouns.

(It really does not matter whether you call it a "demonstrative determiner" or "demonstrative adjective". It boils down to the same thing.)

Solution 2:

This in “Someday this pain will be useful to you.” is a demonstrative adjective (and a determiner.)

Solution 3:

[1] Someday this pain will be useful to you.

[2] Someday this will be useful to you.

No: it's neither an adjective nor a pronoun.

In both [1] and [2] "this" is best analysed as a demonstrative determinative.

In its dependent use [1] i.e. with a following noun, its function is that of determiner.

In its independent use [2] i.e. without a following noun, its function is that of fused determiner-head.