What is the grammatical difference behind "is interesting" and "is interested"?

I am a native English speaker, yet I cannot explain to a non-native speaker why I say:

I am interested in history.

as well as

History is interesting to me.

Why is it "is interesting" when history is the subject, but not when I am?

Solution 1:

Start with the verb form:

History shocks me.

There are two parties to this kind of shocking event: a shock-er, the one doing the shocking (in this case, history); and a shock-ee, the one being shocked (in this case, me). In grammar, the first is called the agent and the second the patient.

English would be very confusing if we had no way to tell which was which! Fortunately we do. When shock is used as a verb, the main hint is the word order: John shocked Kaitlin vs. Kaitlin shocked John. The subject is the agent. The direct object is the patient.

To turn a verb into an adjective, we use different suffixes that help make this distinction. The suffix -ing makes an adjective (called a participle) that describes the agent: shock → shocking, tends to shock people. The suffix -ed makes an adjective (called the past participle) that describes the patient: shock → shocked, startled or upset.

(Many words have irregular past participles: break → broken, not breaked.)

Solution 2:

To expand on the other answers, there is nothing special about interesting vs interested.

• I am interested in X. — X is interesting to me.
• I am excited about X. — X is exciting to me.
• I am worried about X. — X is worrying to me.
• I am horrified by X. — X is horrifying to me.
• I am surprised by X. — X is surprising to me.
• I am puzzled by X. — X is puzzling to me.
• I am amazed by X. — X is amazing to me.

And so on, and so forth. And note how saying "X is interested in me" would be perfectly grammatical. It just wouldn't mean the same thing as "I am interested in X". But I don't see why that should be surprising. At all.

Solution 3:

People are interested; topics are interesting