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In this lesson, we are going to look at the sentence ending “-고 말다.” First of all, please note that this is found more commonly in written Korean than in spoken Korean. “-고 말다” is used to express how a certain action comes to an end or is completed after going through a series of events. This is similar to the sentence ending “-아/어/여 버리다,” but “-고 말다” tends to be more objective and “-아/어/여 버리다” tends to show how the speakers feel about the situation more actively, in addition to being used more commonly in spoken Korean.
Structure:
Verb stem + -고 말다
When you talk about an action in a simple statement and do not provide a lot of background information, using “-고 말다” can be inappropriate. For example, if you want to start a conversation by saying “저 어제 이거 샀어요” (I bought this yesterday), if you say “저 어제 이거 사고 말았어요” instead, you are expecting the listener to already know what kind of situations or previous actions the speaker has gone through before reaching that final state or action.
Examples
1.
잠들다 = to fall asleep
잠들었어요. = I fell asleep.
잠들고 말았어요. = (I was doing other things so I didn’t want to fall asleep but after a while, eventually) I fell asleep.
공부를 하다가 잠들고 말았어요. = I was studying but then I fell asleep.
2. 
감기에 걸리다 = to catch a cold
감기에 걸렸어요. = I caught a cold.
감기에 걸리고 말았어요. = I ended up catching a cold.
As you can see in both examples above, the result is somewhat undesirable. This is not always the case and there is usually less negative feeling on the speaker’s side about the situation when using -고 말다 than -아/어/여 버리다.
Sample Sentences
1. 결국 힘이 다 빠지고 말았어요.
= I ended up using all my energy and became exhausted.
(Understood: You made some efforts to continue without being exhausted, but eventually, after a series of actions, you became exhausted.)
2. 이곳도 사막이 되고 말았어요.
= This place has also turned into a desert.
(Understood: The place was not a desert before, but after going through a series of situations, it has eventually become one, to your disappointment.)
3. 너무 어두워서 머리를 벽에 부딪히고 말았어요.
= It was too dark, so I ended up hitting my head against the wall.
(Understood: You were trying not to get hurt and did your best to find your way around things even though the room was very dark, but eventually, you hit your head against the wall.)
4. 범인을 쫓아갔지만 놓치고 말았어요.
= I went after the criminal, but I lost him. 
(Understood: You made efforts to go after the criminal and catch him, but eventually, you lost him.)
5. 가겠다는 약속을 하고 말았어요.
= I ended up promising to go.
(Understood: You, perhaps, didn’t want to or weren’t supposed to, but you promised someone that you’d go to a place after some talking.)