Determiners are noun modifiers that provide important context to a noun. They are words like 모든, 새, 이, 그, 저. However, when we want to modify nouns with a noun, we sometimes need the marker, 의.
Here are some examples.
오래간만에 동생의 소식을 들었다.
I could hear the news about my brother after a long time.
대통령의 생각에 동의하지 않는다.
I don't agree with the idea of the president.
The news of the year.
의 is the only determiner marker in Korean language. There is no other words. Like always, let's dig deeply about it.
First of all, when 의 should be added after words like 나, 저, 너, they can be changed to 내, 제, 네. And in modern Korean, the pronunciations of ㅔ and ㅐare same, 네 and 내 sometimes cause confusion. In those cases, we clearly say "나의" or "너의". Or we read 네 as [니]
그건 내 책이다.
That's my book.
네 물건은 네가 잘 챙겨.
You should keep your things well.
Secondly, 의 can be skipped a lot in these 3 cases.
1. When it is a possession of a person.
그거 제임스 가방이야 킬리언 가방이야?
Is it the bag of James or Killian?
그거 엄마 거라 손 대면 안 돼.
That's my mother's. So, you shouldn't touch it.
2. When it's a part of big thing.
서울 거리는 붐볐다.
The street in Seoul was crowded.
우리나라 경제 상황이 좋지 않다.
The economy of my country is not good.
철수 엄마는 행복해 보이더라고.
철수's mother looked happy.
In Korean, the phrases like 철수 엄마, 민수 아빠 are used to call adults.
Third, you cannot remove 의 in these cases.
1. When the modified noun has another modification.
서울의 눈 오는 거리는 아름답다.
The street of Seoul in snow is beautiful.
우리나라의 2017년 경제 상황을 봤을 때, 2018년도 그리 좋을 것 같지는 않다.
As we see the economic state of my country in 2017, it seems that it wouldn't be better in 2018.
2. When there is a number in front of 의.
네 자루의 연필.
The four pencils.
세 명의 사람이 있으면 그 중에는 나의 스승이 있다.
If there are 3 people, you can learn something from one of them.
-> It means you can learn something from anyone in the world.
Finally, 의 has a lot of meanings.
For example, 엄마 사진 means
1) 엄마가 찍은 사진(The picture my mother has taken)
2) 엄마를 찍은 사진(The picture that has my mother in it)
3) 엄마가 가지고 있는 사진(The picture that my mother has)
It's hard to determine what it means from only words. You need contexts.