What is the difference between - 내다 and - 버리다 to convey completion of an action?

Question

What is the difference between - 내다 and - 버리다 to convey completion of an action?

For example, would there be any identifiable difference in meaning between "이 책을 읽어냈어요" & "이 책을 읽어버렸어요" ?

Answer 1

My opinion:

1. 내다 is used when the action results in some achievements.

2. 버리다 is used when the action makes the situation irreversible.

책을 읽어냈어요 means you read the book despite some obstacles.

책을 읽어버렸어요 could be interpreted in two ways. It could mean you read the book completely, so you can't make the book unread. It could mean you read some forbidden book and there is no way to come back to the situation before reading.

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Response to comments:

Because the situation becomes irreversible, 버리다 is often used with regrets. But that's not always the case. Here are examples.

숙제는 바로바로 해버리는 것이 좋다. (It's good to do homeworks immediately.)

그는 입이 커서 살구를 한 입에 먹어버릴 수 있다. (His mouth is big so he can eat an apricot with one bite.)

그가 시험에 떨어져 버렸으면 좋겠어. (I hope he fails the exam.)

Answer 2

'-(어) 내다' is an auxilliary verb that means:

앞말이 뜻하는 행동이 스스로의 힘으로 끝내 이루어짐을 나타내는 말. 주로 그 행동이 힘든 과정임을 보일 때 쓴다.

It is used to indicate the action or process of the action has been difficult for you to do or complete.

이 책을 읽어 냈어요 sounds natural only when the book has more than usual number of pages or full of jargons and unknown terms such as medical textbook or science book. I don't think the expression is used broadly and I have never heard anyone use this expression.

'-어 버리다' is also an auxilliary verb that means:

앞말이 나타내는 행동이 이미 끝났음을 나타내는 말. 그 행동이 이루어진 결과, 말하는 이가 아쉬운 감정을 갖게 되었거나 또는 반대로 부담을 덜게 되었음을 나타낼 때 쓴다.

It is used mainly to indicate completion of any action, result of which could be either regretful or relieving (you of some burden in your mind).

For example: 그 책을 읽어 버리고 나니 속이 시원하다. It indicates the completion of reading the book and (as a result) I feel very relieved. In other words, reading the book has become a burden, but after reading it, I feel far better than before.

이 책을 읽어버렸어요 sounds also a little weird. It could only make sense when someone told you not to read the book or you are not supposed to read the book (as it is X-rated or something). I have never heard anyone use this expression, either.

You need to note that the Korean language is very context-dependent and you should not attempt to learn it using just word-for-word translation. Unless it is learned from the right examples with the right context, it will never be easy to learn it properly. Both of the sentences sound unnatural and their usage seems to be very limited.