Complex sentence: When thinking a way to get to the Sky Castle, he accidentally saw the Hien by the bottom of the lake and claim that he'll stay by the Hien to start repairing the airship so it can be used to fly to the Sky Castle.
Split into simpler sentences: When thinking a way to get to the Sky Castle, he spots saw the Hien by the bottom of the lake. Elc decided to stay by the Hien to start repairing the airship so it can be used to fly to the Sky Castle.