Most English adjectives that describe an emotional experience have two possible endings: ing or ed.
- The book interests me. (verb)
- I’m interested in the book. (the experiencer: -ed)
- The book is interesting. (the source of the experience: -ing)
The experiencer is always marked with an -ed adjective, while the source of the experience is marked with an -ing adjective. Generally, inanimate objects can only take -ing.
Animate objects can take either -ing or -ed. Hence, a teacher could be boring if the students are about to fall asleep, or a teacher could be bored if he has taught the same lesson a hundred times already.
Looking at our example again:
- The book [source] interests [emotion] me [experiencer]
- [source] [to be] [emotion]+ing
- [experiencer] [to be] [emotion]+ed
If you are ever in doubt, rephrase your sentence with a verb instead of an adjective; the word order will always tell you whether the adjective should take -ed or -ing.