Hola Emanuel
This sentence:
¡Que llegas tarde!
You will arrive late!
It is as if we were saying "I am warning you that you'll be late!
so we need it in affirmative.
You could also say:
¡Que no llegas a tiempo!
You won't get there on time!
(I am warning you that you will NOT get there on time!)
In this case you need the negative because the warning is about "Not getting there on time".
I hope this clarified it.
Saludos
Inma
