Ok you remember when Doby blocked pass to the Hogwarts Express to save Harry from terrible things in Hogwarts in harry potter 2?how did he know that Harry and Ron will pass the last???????

Truthfully I think I forget, its been forever since i've read that book but I think that dobby stalled them.

I think that maybe he was there, watching them at the station ... maybe invisible if that's possible ... and that's when he saw that Ron and Harry were last, and they went together i think, so he had no choice but to block the entrance for Ron too.

he didn't at the beginning...he was just hidden somewhere near them and when he saw harry and ron being the last left thought it was a great opportunity to keep harry away from hogwarts!