when a shark ______ (appear),a bell rings will tell the people to get out of the water

when a shark <appears>(appear),a bell rings will tell the people to get out of the water
条件状语从句主句是一般将来时，从句用一般现在时。

when a shark appwars(appear),a bell rings will tell the people to get out of the water
当一只鲨鱼出现的时候，铃声将会响起告诉人们是时候离开水面

appears.
用一般现在时.

is appearing