Simplify the following text for a Grade 9 native English-speaking person.
Original: Aang , the current Avatar , must master the four elementsâ '' Air , Water , Earth , and Fireâ '' before defeating the Fire Lord and ending the war .
Simplified: Aang , the current Avatar , must master the four elementsâ '' Air , Water , Earth , and Fireâ '' before win the Fire Lord and ending the war .