"This","that","it" and "the" in English are approximately similar to 이, 그, and 저 in Korean. But there are many differences and they do not have 1:1 mapping.
First of all, in English, “this”(roughly equivalent to "이"), “that”(roughly equivalent to "저", sometimes "그") can be used independently as a pronoun or with other words. In Korean, however, 이/그/저 are rarely used by themselves. In everyday conversation, you would never use them alone.
The meaning and usage of 이/그/저 are following:
이 is very similar to "this" in English.
This book is good : 이 책 좋아요.
Korean language does not have articles. However, when "the" is used as a definite article (usage 10 in
http://www.englishpage.com/articles/a-an-vs-the.htm), i.e.,
when talking about something which is already known to the listener or which has been previously mentioned, introduced, or discussed, "그" can be used to achieve the same goal.
For example, if the listener mentioned about a book and asks your opinion, you can say
그 책 좋아요.: The book is good.
On the other hand, 그 can mean "that", if the referred object is near to the listener.
For example, if the listener brought a book and you feel curious about it, you can say
그 책 뭐에요?: What is that book?
If you want to refer something that is far from you, you can use "저".
For example, if the listener asked you where you found some specific information and you got it from a book which is on the table that is visible but somewhat far, you can say
저 책. (that book over there).
Since 이, 그, and 저 in Korean can only work as modifiers, when you want to say ‘this’ ‘that’ or
‘it’ as pronouns, you have to add the word 거(thing) after these words.
Therefore,
이(this) + 거 =
이거 : this thing, this item, this one
그(the,that) + 거 =
그거 : the thing, the item, that one,
it
저(the, that, that ... over there) + 거 =
저거 : that thing over there
Notice that 그거 also means "it".
거(thing) can be also written or spoken as 것. The meaning is the same and it only makes pronunciation difference. Sometimes 것 can be more natural, but remembering 것 = 거 should be enough for now.
이거 = 이것
그거 = 그것
저거 = 저것
You can form various expressions using 이, 그, or 저 along with other words. For example, if 이/그/저 refers a person, instead of an object, you add 분(polite) or 사람(casual)
이 사람 / 이 분: this person
그 사람 / 그 분: that/the person
저 사람 / 저 분: that person, that person over there
https://quizlet.com/88721317/
