"What is in his pocket?"
Translation:Kio estas en lia poŝo?
I answered the opposite of this question in another post, so I figured I'd cover both sides. :)
The other sentence was "What do you have in your pocket?" and used Kion, not Kio. In the other sentence, it's asking what YOU have in your pocket, you're the subject, and whatever is in your pocket (the one ring to rule them all) is the innocent inanimate OBJECT that you're keeping in your pocket and not sharing.
In this, it's not asking what YOU are keeping in your pocket. It's asking "What is in your pocket?" As in, what is existing in your pocket. In this sentence, YOU don't exist, so you're not the subject.
What is in your pocket? __ is in my pocket. The Ring is in my pocket.
The Ring = the thing doing the existing Is = the action, indicating that it's simply existing and being The Pocket = The poor hapless pocket, the victim of an intruding Ring existing inside of in like some cancer. It is the object that the verb is happening to.
If it gets confusing, I like to think of... what is the "victim" here because that's more direction. A victim is someone or something that an action has happened to. The word object can mean the object of a sentence, but also can mean... an object in your room.
So, if you ever get confused if something is the subject or the object, and it's easy to do, just think, "Who is the victim?" "What is the victim of the action?"
It's all about transitive(vt)/intransitive(vn) verbs. "Havi" for example, is a transitive verb. Each transitive verb require a direct object (in case of EO this results in accusative, in other words it needs kion). "Estas" and a lot of other verbs are intransitive (kio). You must not use a direct object with it. There are also some other verbs that can be both depending on context.
This transitivity thing can be found in a large number of languages. At least the complete gemanic language family, the complete romance language family, the complete slavic language family have this aspect. Once, I really understood it through EO.
In case of EO, one need to remember which verb is (vt) and which is (vn). So is the theory. In fact, one get somedays a feeling for that.