Relation of Energy with Work

Hello all.
This is not a "What is Energy" Thread but rather a question about its relation with work.
I understand that energy has been defined as the ability to do work. So why is change in energy is equal to work? Cant energy be equal to work.

Also when the force field is doing work on a object, will the work done on the object by field be equal to the work that the object can do later?
Thanks