In my three years of teaching in Vietnam and Thailand, I’ve noticed an interesting pattern of mistakes. I often hear things like:
I didn’t like that movie; I felt boring.
I’m very interesting in that topic.
This test was hard. I was really confusing.
In each case, the student is using an adjective, but he’s mixing up the endings. (In these examples, the adjectives should take the –ed ending, not the –ing ending.) Why?
Generally, when we describe our feelings or state of mind, we use the –ed (“participle”) ending.
We can add detail about why we feel this way, but we’re still describing the feeling we have, so the –ed ending remains the same.
I’m bored by this novel.
I’m interested in Roman history.
I’m confused by physics.
When we switch from describing our state of mind to describing the thing that’s affecting us, however - novels, history, physics - we use the –ing ending.
I find anthropology class boring.
Futurama is an interesting show.
Quantum mechanics is so confusing!
In each of these cases, we’re no longer talking about how we feel (not exactly, anyway). We’re talking about something that’s out there in the world (a class, a TV show, a topic), and we’re saying that that thing does something to us.
The lecture was boring; I felt bored.
But Wait! That’s Not All...
Okay, so here’s the really confusing part. It’s also possible to describe yourself using adjectives with –ing endings:
I’m such an interesting guy!
My mind is so confusing.
The key here is that in these cases, you’re no longer describing a feeling. You’re describing yourself as a thing (a person, a guy, a mind) that has effects (just like the classes, TV shows, and topics above). So, back to the –ing ending.
I’m an interesting dude! I make other people feel interested in me.
When you’re talking about your own feelings, use –ed. When you’re talking about something that has an effect on something else, use –ing.
I was confused for two straight hours. Why? Because the play was confusing!
One more wrinkle: You can also describe things in the world as “confused”:
That episode of The Sopranos was confused.
In this case, you’re not describing the show's effect on you. Rather, you’re describing the show’s state of mind - its 'feelings,' so to speak. (To be precise, you're personifying or anthropomorphizing.) You’re saying that the show was all over the place – that it didn’t know what it was trying to say.