English has two kinds of participial adjectives:
ending in -ed:
I am disappointed.
ending in -ing:
They seem to mean different things. How I can explain the difference between them to my students?
To say that you are disappointed describes a feeling that you have. For example, "I am disappointed because the Giants lost."
To say that something is disappointing means that it causes (or would most likely cause) someone to feel disappointed. For example, "The Giants' loss was disappointing."
I was disappointed because the act was disappointing.