This, that and what

English splits the world into near me, this, and not near me, that. Korean adds a third term by measuring from the listener as well: 이 is near the speaker, 그 is near the listener, and 저 is away from both. The three attach to 거, the spoken form of 것 (geot), 'thing', to make the pointing words. Because 그 means 'near you, or already in our heads', it also covers things that are out of sight but under discussion, a job English gives to that and it.

Two markers are at work here. 이/가 marks the subject, and it sits naturally on the thing you are asking about, because that is new information: 이게 뭐예요? The answer switches to 은/는, because by then the thing is the topic you are both looking at: 그건 김치예요. Like 은/는 on day 3, the subject marker changes with the sound before it, 이 after a consonant and 가 after a vowel. In speech both markers fuse with the pointing words, giving 이게 and 그건, and a learner who knows only the long forms will not recognise them.

이거 is really two words, 이 plus 거, and 이, 그 and 저 go in front of any noun in exactly the same way, like this and that in English. So once you know the three, 이 사람, 그 가게 and 저 사람 need nothing new beyond the nouns. The one trap is 저: in front of a noun or 거 it means over there, while standing alone before 는 it means I. Korean has many of these short look-alikes, and word order sorts them out almost every time.