Classifier alone when the noun is clear: cái này, con kia

When it's clear what you mean, keep the classifier and drop the noun: cái này (this one), cái đó (that one, the one we're talking about), cái kia (that one over there), con này (this one, an animal), cuốn nào? (which one, a book). The classifier, the small counting word, stands in for the noun, much as "one" does in English.

This is how you point and buy: Cho em cái này. I'll have this one. Chị lấy cái nào? Which one would you like? Use the classifier the noun would take: trái này for fruit (in Hà Nội quả này), con kia for a fish or a puppy.

Này, đó, kia and nào can't stand on their own as "this one", "that one" or "which one". The classifier has to stay: muốn cái này, never muốn này. Đây doesn't work here either; it's for introducing (Đây là …), not for "this one".

Numbers work the same way. Once you know what's being counted, the number and the classifier are enough: Cho anh hai cái. Two, please. Em lấy ba trái. I'll take three.

When the thing isn't clear yet, say the noun: cái áo này (this shirt).