kibodeuga gapjagi an dwaeseo mauseuroman munseoreul yeoreosseo.

Because the keyboard is being presented as the thing that wasn’t working.

• 키보드가 안 되다 = the keyboard doesn’t work
• Here, 가 marks 키보드 as the subject of 안 되다

If you used 키보드를, that would make keyboard the object of another verb, which is not what is happening here. The sentence is not saying someone did something to the keyboard; it is saying the keyboard itself failed.

안 돼서 comes from 안 되다 + -아서/어서.

• 되다 can mean to work, to function, or to be possible, depending on context.
• 안 되다 here means to not work
• -아서/어서 connects clauses and often means because, so, or and then

So 키보드가 갑자기 안 돼서 means something like:

• because the keyboard suddenly stopped working
• the keyboard suddenly didn’t work, so...

In this sentence, it gives the reason for the next action.

돼서 is the contracted, very common spoken and written form of 되어서.

• 되다 → 되어 → 돼
• so 되어서 → 돼서

Both are correct, but 돼서 is much more natural in everyday Korean.

갑자기 means suddenly.

It modifies the idea that the keyboard stopped working:

• 키보드가 갑자기 안 돼서 = because the keyboard suddenly stopped working

Korean adverbs like 갑자기 are often placed before the verb or verb phrase they modify. Its placement here is very natural.

Because 안 되다 is the normal expression for something not functioning or not working properly.

• 키보드가 안 돼 = the keyboard doesn’t work
• 못 하다 usually means cannot do something
• 못 되다 is not the right expression here

A useful distinction:

• 안 되다 = the thing itself doesn’t work / it’s not possible
• 못 + verb = someone is unable to do the action

For example:

• 키보드가 안 돼요. = The keyboard isn’t working.
• 키보드가 없어서 타자를 못 쳐요. = I can’t type because I don’t have a keyboard.

마우스로만 is made of:

• 마우스 = mouse
• -로 = by means of / with / using
• 만 = only

So 마우스로만 means:

• using only the mouse
• with only the mouse

It emphasizes that the speaker could not use the keyboard and had to rely exclusively on the mouse.

Because each particle adds a different meaning:

• -로 shows the means/tool
• 만 adds the idea of only

So:

• 마우스로 = with the mouse / using the mouse
• 마우스로만 = using only the mouse

This stacking is very common in Korean. Other examples:

• 손으로만 했어 = I did it only with my hands
• 한국어로만 말해 = Speak only in Korean

Yes. 열다 is the normal verb for opening many things, including files, documents, doors, apps, and windows.

• 문서를 열다 = to open a document
• 파일을 열다 = to open a file
• 창을 열다 = to open a window

So 마우스로만 문서를 열었어 means the speaker opened the document by clicking around with the mouse.

Because Korean often omits subjects when they are understood from context.

Even though English usually needs I, Korean does not have to say it if it is obvious who did the action. In this sentence, once we hear:

• the keyboard stopped working
• opened the document using only the mouse

it is natural to understand that I am the one speaking about my own action.

A fuller version could be:

• 나는 키보드가 갑자기 안 돼서 마우스로만 문서를 열었어.

But in everyday Korean, leaving out 나는 is very common.

Here, -어서 connects the first situation to the second one as a reason/cause.

So the flow is:

• the keyboard suddenly didn’t work
• so I opened the document using only the mouse

In many sentences, -아서/어서 can mean:

• because
• so
• sometimes simply a natural sequence of events

In this sentence, the causal meaning is especially strong because the second action is clearly a result of the first problem.

열었어 is the informal, casual past form of 열다.

Breakdown:

• 열다 = to open
• 열었어 = opened / I opened

This style is commonly used:

• with friends
• with people younger than the speaker
• in casual conversation
• in diary-like or relaxed speech

A more polite version would be:

• 키보드가 갑자기 안 돼서 마우스로만 문서를 열었어요.

By itself, 안 되다 just means doesn’t work / isn’t working. But in this sentence, because of 갑자기 (suddenly), the natural interpretation is:

• it suddenly stopped working
• it suddenly wasn’t working

So the sentence suggests a change of state, not just a general fact about keyboards.

No, it sounds quite natural. It is a very normal everyday Korean sentence.

A native speaker would easily understand:

• there was a computer-related problem
• the keyboard failed unexpectedly
• the speaker had to use only the mouse to open a document

The only thing left unstated is the broader situation, but Korean often leaves that kind of background implied.