Yes I think you already know it. 이 그 저 should be followed by a noun word. 이거 그거 저거 already include the noun word.

For example, 이 모자 means "this hat".

Imagine that you go to clothes store and looking for a hat. You find a hat what you like and you want to ask how much is it. Then you say 이거 얼마에요? At this situation you are referring the hat you'd like to buy. We don't say 이거 모자 얼마에요?.

You have to say either these two sentences.

이 모자 얼마에요? Or 이거 얼마에요?

Do you get it? :)

I have been following this lesson but i still dont understand in wich situatuion u use the modifyer (이, 저, 그,) and when you combine them with 거/것 (이거, 저거, 그거)

This is the examples i got in the lesson:

이 [i] = this (near you) (adjective)

이 커피 [i keo-pi] = this coffee

이 컴퓨터 [i keom-pyu-teo] = this computer

이 아이스크림 [i a-i-seu-keu-rim] = this ice-cream

이 호텔 [i ho-tel] = this hotel

이 택시 [i taek-si] = this taxi

이 버스 [i beo-seu] = this bus

이 카메라 [i ka-me-ra] = this camera

* If you know how to say “I” in polite formal Korean, it’s the same thing. It sounds the same. But you don’t have to worry about being confused, because “저” is a modifier which modifies a different word, so it is always followed by a noun. The word for “I”, “저”, is followed by particles, verbs, etc. They are used in completely different contexts.

저 마이크 [jeo ma-i-keu] = that microphone over there

저 테이블 [jeo te-i-beul] = that table over there

저 레스토랑 [jeo rae-seu-to-rang] = that restaurant over there

* “그 [geu]” and “저 [jeo]” are both used for referring to something that is far away from the speaker, but “그” is used when you are talking to the other person and that object is near that other person and far away from you, and “저” is used for referring to something that is far away from both you and the other person.

거 [geo] = 것 [geot] = thing, item, stuff, fact

So... 

이 [i] = this 

이 + 것 = 이것 [i-geot] or 이거 [i-geo] = this thing, this item, this one

그 [geu] = the, that

그 + 것 = 그것 [geu-geot] or 그거 [geu-geo] = the thing, the item, that one, it

저 [ jeo] = the, that, that ... over there

저 + 것 = 저것 [jeo-geot] or 저거 [jeo-geo] = that thing over there

You can form various expressions using 이, 그, or 저 along with other words.

Examples:

사람 [sa-ram] means a person.

이 사람 [i sa-ram] = this person, this man here, this lady here, he, she

그 사람 [geu sa-ram] = the person, that person, he, she

저 사람 [jeo sa-ram] = that person over there, he, sh

YES IT IS! It's simple grammar.

Oh ok so like this:

이 커피 [i keo-pi] = this coffee

이거 커피예요. [i-geo keo-pi-ye-yo] = This is coffee.

Whena word "is someting" then its 이거? Like: "this is" coffee

ㅋㅋㅋ I don't understand your question. Ask with some example would be helpful.