What is the difference between -지 않다, -지 못하다, and -ㄹ/을 수 없다?

Question

One aspect of Korean that seems to cause issues for beginners is how to correctly say they will not/cannot do something. (Personally, I feel I use them correctly, but this is a really common issue I came across when teaching beginners Korean, hence my asking the question for the general public).

What are the proper usages for the endings -지 않다 (지 아니하다), -지 못하다, and -ㄹ/을 수 없다?

For example, if I want to tell a coworker that I have not eaten yet because I was too busy, should I say "너무 바빠서 아직 먹지 못 했어요" or should I say "너무 바빠서 아직 먹지 않았어요"?

(Alternatively in the short form, the sentences above can be "너무 바빠서 아직 못 먹었어요" and "너무 바빠서 아직 안 먹었어요")

How does the -ㄹ/을 수 없다 ending play into this conversation of negating a verb, and when is it appropriate?

Answer 1

The little nuances could make for a very long and detailed answer, but the general rules are along these lines:

• ~지 않다 (And by extension, 안 하다)
  • On a 동사/Action Verb
    • One doesn't do something, regardless of whether it's possible or not.
    • Very often implies (with strength of the implication depending on context) deliberatly not doing something. But it doesn't automatically confer actual deliberateness in all cases.

      저는 가지 않았어요 => "I didn't go"

  • (on a 형용사/Adjective)
    • One isn't something, again regardless of whether it is possible to be .

      저는 똑똑하지 않아요 => "I am not smart."

• ~지 못하다 (And by extension, 못 하다)

  • One can't do/be something.

  • Does not imply that the action is impossible, rather, it implies that the action couldn't be done perhaps due to circumstances, but may otherwise be possible.

    못 들었어요 => "I couldn't/didn't hear you."

    못 갈 거예요 => "I won't be able to go."

• ~ㄹ 수 없다
  • This implies that something is actually impossible (or if context is specific, is impossible within that context.)

    인간은 날 수 없어요 => Humans cannot fly.

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There is at least one important extra nuance to note, however. Many words, such as 보다 or 듣다 will have stronger implications of deliberateness when using between ~지 않다 instead of ~지 못하다:

Also, 듣다 specifically (and surely many other words that are hard to summarize) has a pretty important distinction in meaning depending on context. It's hard to get the feeling when translating, but for a general idea:

듣지 않았어요 => I didn't listen to it. (Sort of strong implication that this is intentional or by oversight)

듣지 못했어요 => I couldn't hear it.

Answer 2

안 = don't (않 = 안하(다)/아니하(다), 안 = 아니 )

못 = can't

없다 = be not

1. 안, 못 with/without space.

And there are also verbs with 안, 못. for example,

안되다 = poor

안 되다 = doesn't work

못하다 = be not good at

못 하다 = can't do

못생기다 = be ugly

못 생기다 = can't create

2. 있다 / 없다

(presence)

There is/are ~ = ~이/가 있다.

S be-verb ~ = S이/가 있다.

(possibility / ability)

be able to / can / could / may / might = -ㄹ 수 있다

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