-기 vs -음 vs -은/는/을것

Question

There doesn't seem to be a full elaboration of the nuances and uses between these three anywhere on the internet (in English for English → Korean learners). So I would like to ask here if someone more knowledgeable could provide such a thing.

Answer 1

Generally speaking,

• Action verb + -ㅁ : a nominal ending that derives a new word, a generic or abstract noun from the action. For exmaple, 살- (to live) + -ㅁ = 삶 (life).
• State verb (adjective) + -기 : a nominal ending but mostly limited to the scale or degree of the state. While it can mean a simple value-neutral scale, it usually indicates the positive side of the scale, especially when used in a causal context (-에, 때문, 위하-, etc) or with degree adverbs.

  • 이 문제 어렵기가 장난이다.
    The difficulty of the problem is a joke. # meaning it's easy, although the word 어렵- is used.
  • 이렇게 쉽기도 어렵다.
    Being this easy is difficult/ rare.
  • 문제가 어렵기 때문에 오래 걸렸다.
    Because of the difficulty, it took a long time. # meaning high difficulty.

• Action verb + -기 : doing that action, similar to gerund in English. For exmaple, 살- (to live) + -기 = 살기 (living).
• State verb + -ㅁ : being in that state, similar to gerund but for a static property.
• Any verb + -ㅆ- + 기 : gerund of a past event or state, ONLY used in a causal context.

  • 네가 먹었기에 하나 밖에 안 남았다.
    Because you ate, there's only one left.
  • 시험이 어려웠기 때문에 난 덕을 봤다.
    Because of the test being difficult, I benefited.

• Any verb + -ㅆ- + 음 : gerund of a past event or state, NOT used in a causal context

  • 네가 먹었음이 틀림없다.
    It is clear that you ate it.
  • *시험이 어려웠음 때문에 난 덕을 봤다.

Except for the top case (action + ㅁ), these nominalizations usually denote a particular action or state.

-ㄴ (것), -ㄹ (것), -는 (것) (and -던 (것)) are attributive endings and function similarly to relative clauses in English. See https://korean.stackexchange.com/a/1810/358 for differences between them. Compared to the above verb nominalization, attributive endings have wider bandwidth for various conjugation patterns that can throw in more information about the verb (for example, combination with -ㅆ- alters the meaning and usage of -ㅁ and -기 as above). Also, they should have a noun that is doing (or affected by) the action or being the state, and besides (probably) the most generic light noun 것, they can also go with different light nouns (such as 터, 지, 바, 뿐, 만큼, and more) to carry different (sometimes quite nuanced) meanings.