Ну так они же учатся писать числа. Любой может написать 2,3,4,5. А как писать и произносить?
When you are dealing with specific things, you have to use an article: the definite article "the" for a specific single thing ("a banana") or specific plural things ("the bananas are yellow"), and the indefinite article "a" for a single thing which is specifically identified ("there is a banana on the floor - watch out!") but which is not as specific as "the" banana ("There is a banana on the floor!" "Yes, it is the banana I bought today; apparently, it fell out of the supermarket bag when I walked into the kitchen.") You can get more specific while also generalizing without getting entirely abstract: "The banana I am giving you is a green banana; you must allow it to ripen."
When you talk about things in general, abstract terms, you usually use plurals without an article: "Bananas are a kind of fruit."
More formal English sometimes uses articles to express such generalities "A/The banana is a fruit." But that is very formal and not encounter with any frequency.
Note how I began this comment with an abstract generality without an article "When you are dealing with specific things...." The generality is "specific things" - completely unidentifiable, very vague and general, with the only qualification being that they are specific when actually dealt with.