bang jeongureul sae jeonguro gara kkiwosseoyo.

In Korean, two nouns are often placed together to make a compound noun, so 방 전구 can mean the room bulb / the bulb in the room.

• 방 전구 = a noun-noun combination
• 방의 전구 = the room’s bulb or the bulb of the room

Both are possible, but the version without 의 often sounds more compact and natural in everyday speech.

Also, Korean often prefers this kind of compressed noun phrase where English would use of or a longer description.

They have different jobs in the sentence.

• 전구를 marks the thing being changed/replaced.
• 새 전구로 marks what it was changed to / with.

So:

• 방 전구를 = the room bulb (direct object)
• 새 전구로 = with a new bulb / into a new bulb

The particle 로 often shows:

• a tool or means
• a direction
• a resulting state
• what something is changed into

Here it is the replacement item.

Here, 새 전구로 means something like:

• with a new bulb
• for a new bulb
• to a new bulb

In English, we usually say replace X with Y. Korean often uses X를 Y로 바꾸다 / 갈다 / 갈아 끼우다.

So the structure is basically:

• old thing + 를
• new thing + 로
• replace verb

That is why the second noun uses 로, not 를.

갈아 끼우다 is a very natural expression for replacing something that is fitted into a slot or device.

It combines two ideas:

• 갈다 = to change/replace
• 끼우다 = to insert, fit in, put into place

So 갈아 끼우다 gives the sense of:

1. taking the old one out
2. putting the new one in

That is especially appropriate for things like:

• light bulbs
• batteries
• filters
• parts that get inserted into something

If you only use 갈다, the meaning is more general: to change/replace.
갈아 끼우다 is more vivid and specific.

This is because Korean often uses -아/어 in verb combinations where the first action leads directly into the second one.

So:

• 갈아 끼우다 is a common set expression
• it feels like one connected action: replace-and-insert

If you said 갈고 끼웠어요, it would sound more like two separately listed actions:

• changed it,
• and inserted it

That is not impossible in theory, but 갈아 끼우다 is the normal expression here.

끼웠어요 is:

• past tense
• polite speech in the standard -어요 style

So it is the kind of form you would use in everyday polite conversation.

Breakdown:

• 끼우다 = base verb
• 끼웠- = past stem
• 어요 = polite ending

So 갈아 끼웠어요 means replaced it in a polite, conversational way.

Yes, the subject is omitted.

This is extremely common in Korean. If the speaker or subject is already understood from context, Korean usually leaves it out.

So depending on context, this could mean:

• I replaced the room bulb
• He replaced the room bulb
• We replaced the room bulb

Korean does this much more often than English.

Usually 새 전구 strongly suggests a new bulb, and often a brand-new one.

But in practice, it mainly contrasts with the old bulb and means the replacement bulb. In many situations, the important point is simply that the old bulb was removed and another one was put in.

So the nuance is usually:

• old bulb out
• new/replacement bulb in

Yes, that is grammatically fine.

Using 방의 전구 is a little more explicit because it shows the relationship with 의. It may sound a bit fuller or more formal than 방 전구.

Possible versions include:

• 방 전구를 새 전구로 갈아 끼웠어요.
• 방의 전구를 새 전구로 갈아 끼웠어요.
• 방에 있는 전구를 새 전구로 갈아 끼웠어요.

They are all understandable, but the nuance and style are slightly different.

Yes. 갈아 끼우다 is often used for things that are physically inserted or fitted into something.

For example:

• batteries
• filters
• machine parts
• printer cartridges
• some kinds of components

So it is a good verb to learn as a pattern: remove the old one and fit in a new one.

That is why it sounds very natural with 전구.