The Whomping Willow was planted to protect the secret tunnel into the Shrieking Shack. Remus Lupin would use the Shrieking Shack to have a safe place to turn into a werewolf when he was a student at Hogwarts.
25) Who caused Harry many problems while trying to "help" him in his second year at Hogwarts?
A house elf can only be freed if their master presents them with clothing. Harry gives Lucius Malfoy Tom Riddle's diary wrapped in a sock and Lucius throws the sock aside. Dobby catches the sock which sets him free.