1. Because Arthur is Uther's pay for Merlin's service. (And another version, that Merlin sad, that child should be raised in secret place)
2. Artur grow up with sir Ector.
3. The message was on the sword.
4. Every knight tried to pull the sword from the stone.
5. He was 16 years old.
6. the Lady of the Lake gave the sword to king Arthur.
P.S. ответы могут быть не совсем правильными, т.к. в вашем учебнике 1 текст, а в других источниках - другой.