Classifier for one particular thing, bare noun for things in general: con chó này, chó

A classifier does more than count. Without one, a noun means that kind of thing in general: Tôi thích chó. I like dogs. With one, it singles out an individual, usually one you can see or both have in mind: Con chó này dễ thương quá. This dog is so cute. Con mèo đâu rồi? Where's the cat?

So likes, habits and general questions use the bare noun: Em không ăn thịt heo (I don't eat pork), Anh có nuôi chó không? (Do you keep a dog?). That question is about dogs in general, not a particular one.

When you mean one particular thing, especially one you're pointing at or have just mentioned, put the classifier in: cái áo này, con cá đó. After a number it's usually needed whichever you mean: ba con chó (words that are units themselves, like ngày, take none: ba ngày). Several particular ones keep the classifier too, with mấy or những in front: mấy con chó này (these dogs).

With everyday nouns like nhà, phòng or xe, the classifier is often left out even for a particular one, as long as it's clear: Phòng này rộng. So take it as a strong habit rather than a strict rule: the classifier makes the thing particular, and the bare noun keeps it general.