Hello Teachers

Sample sentence : I've never seen a girl this/that/so pretty.

What I like to know is the grammatical interpretation of the adjective phrase.
Do you take it as a shortened form of "a girl who is this/that/so pretty"?
Or is it some post-nominal adjectival phrase already grammaticalized?

Hard to tell, Paco; I can see it both ways:

(1) as the reduced postmodifying clause-- 'I've never seen a girl (who is) this/that/so pretty'.
(2) as an adjective phrase postponed to create end focus-- 'I've never seen this/that/so pretty a girl'--> '...a girl this/that/so pretty'.

Hopefully, someone else has an idea brighter than mine.
Mister Micawber

Thank you for the kind reply. So I would like to learn those "that/this/so adjective" phrases as idiomatic phrases grammatically exceptional in that they come after nouns, although I might interpret them as reduced forms of adjectival relative clauses. By the way I prefer them to the construct "such a/an adjective noun". "Such a/an adjective noun" sounds to me a little ambiguous because we are uncertain about whether "such" modifies the adjective or the whole nominal phrase "a/an adjective noun". Anyway I would like to say again "thank you" and "Merry Christmas to you!"