I think its like english - i constantly call cups glasses and vis versa ...
probably because "glasses" can be misconstrued to be synonymous with spectacles - possibly to mitigate confusion, "cups" was used in the exercise, and so the explanation shows implicitly that "cups" and "glasses" can be used interchangeably, so now the student understands that, as in English, "cups" and "glasses" can be interchanged in Italian as well.
Hope this clears up some confusion. *)
Isn't "gli" allowed too for this? I'm confused. When do you use "gli" and when do you use "i"?
You use gli in the plural when you'd use lo in the singular, and the same goes for i and il; in this case, "il bicchiere" - "i bicchieri". Of course there are exceptions as with anything, but they are few.
you also use gli when it is a masculine plural that starts with a vowel.. gli uccelli