"The dog of the third boy."
Translation:Câinele celui de-al treilea băiat.
Really? Can you explain then? Why is "cainele al treilea baiat" wrong?
The grammar below calls the function of celui here "a support element for the ordinal numerals, especially in order to express the case functions".
So basically, al treilea băiat cannot carry (enough of) the genitive information - even though băiat on its own could be genitive - and the preposition de seemingly doesn't fix it, thus celui to the rescue... but I'm not sure, if celui is mandatory or if there are other workarounds.
see section 3.2.2. in
No, actually I was being sarcastic.... I didn't get it. I guess the reason is that the way you wrote it (likely what I first wrote, too) doesn't show that the dog belongs to the boy.
Gene...I agree that it could be. And i suppose Duolingo should allow it. But it's not the only way to say it... as it might make a complicated sentence even a bit more difficult for many to unravel.