I have always had difficulties understanding modifiers in Korean, reading lessons about this topic for once is not enough. I guess you really have to read over and over to fully understand its use. This is were I struggled the most.
Going back to lessons on simple modifier, now it seems more clear to me it’s use. When used with an adjective, it directly modifies the noun in front of it. So this means descriptive verbs (non action or non processive types) directly modifies the noun in front of it when it takes t. is form. An example would be from this sentence: 침대이 커요 (Chimdae.i keoyo) — which means ‘the bed is big’ can be expressed in a different manner by using this simple modifier. 큰 침대 (Keun chimdae) would then mean the bed that is big. In this form the phrase can be used to create a little more advance sentence: 큰 침대을 사고 싶어요 (Keun chimdaereul sago shipeoyo), this means I want to buy a big bed.
This modifier works differently when attached to an action word or processive verbs. As mentioned by one commenter (Cristo), it has a past meaning when used with processive verbs. Here are some examples:
- 걸은 사람 (Goreun saram) – the person who walked
- 쓴 편지 (Sseun pyeonji) – the letter that (I) wrote
- 기다린 어머니 (Kidarin eomeoni) – the mother who waited
Also got a tip that whenever processive verbs are used in this form, there is no need to transform the verb into past tense since the use of the modifier already signifies that that action has been done already. Therefore it would be impossible to see 걸었언 사람 (Keoreosseon saram) since 걸은 사람 would already meant a person who walked.