Is it correct to say that determinatives cannot be modifiers? Can they be considered a kind of modifier? Or are they definitively distinct functions?
Determinatives commonly function as determiners in noun phrase structure, but they can have other functions too. For example, the determinative "the" is a determiner in
The younger son had died, but a modifier in
I feel all the better for my holiday.
And the demonstrative determinative "that" is a determiner in
Who is that tall guy over there?, but a modifier in
It wasn’t that great.
Similarly, the cardinal number determinative "three" is a determiner in
Ed has three cars, but a predicative complement in
We are three in number.
Another question if I may—how can our be considered a pronoun when it isn't, and can never be, a noun phrase?
Why not? "Our" is a noun phrase in
No one objected to our joining the party, where it is subject -- a function normally performed by a noun phrase.
And it can be coordinated with a noun phrase, as in
We did it without our or the manager’s approval.
Moreover, I don’t think anyone would claim that the independent form "ours" is anything other than a noun phrase in, for example,
Kim is a good friend of ours, where "ours" is the object of a preposition, again a function normally performed by a noun phrase.
