是主语,
they are able to earn 这个才是定语,修饰the amount of money
他们能够赚到的钱的数量取决于他们的工作量the amount of money 做定语从句的宾语吗？对的，还原回去就是 they are able to earn the amount of money ，所以充当宾语没错to earn 是目的状语吗？