hi Sham, 이/그/저 are articles which require noun to follow like a/an/the in English. 이거/그거/저거 are pronouns like it/that/she/he. Ex) 이 사람 (this person), 그 사람 (the person), 저 사람 (that person) 이거 (this), 그거 (that), 저거 (that) For more examples... 이 음식 먹어봐. (Try this food.) 이거 먹어봐. (Try this.) 저 사람 봐봐! (Look at that guy!) 저거 봐봐! (Look at that!) 그 종이에 적어. (Write down on that paper.) 그거에 적어. (Write down on that.) Hope you got the point..

1. 이 It is used to indicate the person or thing that is closer to a speaker than a listener. 2. 그 It is used to indicate the person or thing that is closer to a listener than a speaker. 3. 저 It is used to indicate the person or thing that is distant from a listener or speaker. 4. –거 -거 is the colloquial of –것. This is dictionary definition. 이거, 저거, 그거 = 이것, 저것, 그것. These have same meaning. But sometimes, the postposition used after to them is different. For example, 이거, 저거, 그거 + <<를>> ex)나는 이거를 한가운데로 옮겼다. I moved it to the center. 이것, 저것, 그것 + <<을>> ex)나는 이것을 한가운데로 옮겼다. I moved it to the center.