This is related to the question here
The answer posted by Gaeguri seems to be what I was looking for, but I wanted some more information, and found this dictionary definition
라 1. 서술적
게으름 피우다간 낙제할~ If you are idle, you will flunk, I warn you
그는 아직 어린애~ 그 말을 이해할 수 없다 He is too young to understand it
그는 내 아들이 아니~ 내 조카다 He is not my son, but my nephew
Only sentence 1 seems to have the sense as explained by Gaeguri in the answer to the other question (and it sounds like something an adult would say to the child) but the other examples don't seem to have the same usage of 라
How is the usage of ~라 as used in the first sentence similar (or related) to it's usage in the other sentences?
Also, what does 서술적 mean?