What is the difference between article and determiner?

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[@soumikkumar3244]

Informative video . Thanks man ❣️

[@R.kuntal-mr3fi]

Nice sir

[@ubuenglish]

Regarding the sentences:
This statement sounds reasonable.
This sounds reasonable.
According to generative grammar theory, the word 'this' is the head word because the noun 'statement" can be removed from the sentence.
It seems evident, to me at least, that the fuction of the word 'this' changes in these sentences.
In the first sentence, 'this' functions as a specific or definite determiner, however, in the second is seems more like a pronoun because it is referencing context where the referent to 'this' is established. It is a noun only through its reference to previousy established noun phrase.