How did Malfoy know that Harry was on the Train?
In this case, Malfoy's observations of Harry's foot catching in the door and his past experiences with Harry allowed him to recognize him on the train.
Why did Snape kill Dumbledore?
Do you truly believe that the use of magic is a moral pursuit? Do you think that it is right to wield such power over the natural world and over other people's lives?
Why did Viktor Krum Catch the Snitch in the Quidditch World Cup Final?
Charles, think about it. It's like trying to catch a bus that's already left the station. It's not happening.