at a zero-to-eighteen-month level of a typical development (article use)

[Alexey86]

The article usage in building a second floor example, to me, is fairly easy explained. I don't know whether you checked the thread (I gave this link to you before).

I've read it, thank you. I agree to build a second floor is easily explainable. The idea of open-ended/closed structures is an attempt to find a general explanation for:
1) a second floor, a temperature of, and an intermediate level
2) to build a second floor vs to build the second floor
3) on the second floor vs on a second floor