When do you change jobs in maple story

U change into ur 2nd job (require first job) at lv 30 then u go 3rd job at lv70 (require 2nd job? then u go to 4th job at lv 120 (require 3rd job) but you will need to train extremely hard.
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You don't really change 'jobs' but you rank up in that job type (Job Advancements). Ex: 1st is Magician, 2nd at level 30 is Fire/Poison Wizard, Ice/Lightning Wizard, or Cleric, 3rd at Level 70 you rank up from the type you chose at level 30. Ex: Cleric at level 30-70, 70-120 you become a Priest. From 120 (4th job) and up, you are a Bishop. To do Job Advancements, you need to do specific quests or tasks at level 30 (Victoria Island in the town your job is at), 70 (El Nath) and 120 (Leafre).