sae jeongureul sa wasseunikka jigeum gara kkiumyeon dwae.

사 왔으니까 comes from 사 오다, which means to buy and bring back / buy and come with.

So:

• 샀으니까 = because (someone) bought it
• 사 왔으니까 = because (someone) bought it and brought it here / back

In this sentence, that extra 오다 nuance matters: the new bulb is not just purchased somewhere; it is now available here, so replacing it is possible.

-으니까 means because / since, and it gives the reason for what follows.

Here, the structure is:

• 새 전구를 사 왔으니까 = since we bought and brought a new bulb
• 지금 갈아 끼우면 돼 = we can just replace it now / it’ll be fine if we replace it now

A useful nuance: -으니까 is very common when the speaker gives a reason and then follows it with a suggestion, conclusion, permission, or decision. That fits this sentence well.

It sounds like:

• Since we have the new bulb now, we can replace it.

새 means new and directly modifies a noun.

So:

• 새 전구 = a new bulb

It is similar in meaning to 새로운 전구, but 새 전구 is more natural and common for ordinary physical objects like clothes, phones, bulbs, shoes, and so on.

A good way to think about it:

• 새 = the usual everyday determiner before a noun
• 새로운 = more descriptive, sometimes a bit more literary or abstract

So in this sentence, 새 전구 is exactly what you would expect.

갈아 끼우다 is a very natural verb for replacing something like a bulb, battery, fuse, or similar part.

It combines these ideas:

• 갈다 = to change / replace
• 끼우다 = to insert / fit in / put into a slot or opening

So 갈아 끼우다 literally gives the sense of:

• take the old one out and put a new one in

That is why it works especially well for a light bulb. It is more specific than just saying change.

The object is omitted because it is already obvious from context.

Earlier in the sentence, the speaker mentioned 새 전구. Because of that, Korean does not need to repeat 전구를 again in the second part.

So the full idea is understood as something like:

• (전구를) 지금 갈아 끼우면 돼
• It’s fine if we replace the bulb now

This kind of omission is extremely common in Korean. If the listener can easily figure out the object, Korean often leaves it out.

-면 돼 literally means something like:

• if you do X, it becomes okay
• if you do X, that works
• you just need to do X
• you can do X

Here, 돼 comes from 되다 (to become / to work / to be okay).

So:

• 갈아 끼우면 돼 = it’ll be fine if you replace it
• more naturally: you can just replace it now or all you need to do now is replace it

This pattern is very common in everyday Korean and often has a practical, problem-solving tone.

Korean often leaves out the subject when it is obvious from context.

This sentence does not explicitly say who bought the bulb or who will replace it. Depending on the situation, the hidden subject could be:

• 내가 = I
• 우리가 = we
• 네가 = you

For example, in context it could mean:

• I bought a new bulb, so we can replace it now.
• We bought a new bulb, so you can replace it now.

Korean is much more comfortable than English with leaving subjects unstated.

Because the two actions are at different stages:

• 사 왔- = the buying-and-bringing action is already completed
• 갈아 끼우면 = the replacing has not happened yet; it is what can be done now

So the sentence naturally sets up this timeline:

1. A new bulb was bought and brought here.
2. Therefore, replacing it is now possible.

That is why the first verb is past and the second part is not.

Yes, 바꾸다 can sometimes work because it means to change / replace, but 갈아 끼우다 is more specific and more vivid here.

Compare:

• 전구를 바꾸다 = change the light bulb
• 전구를 갈아 끼우다 = replace the bulb by taking it out and fitting in a new one

For a light bulb, 갈아 끼우다 sounds especially natural because it highlights the physical swap.

So 바꾸다 is possible, but 갈아 끼우다 is more precise.

Usually it means now / at this point, and the exact force depends on context.

In this sentence, it probably means:

• now that we have the new bulb
• at this point, we can replace it

It may sound immediate, but it does not have to mean this exact second. It often just marks that the situation has changed and the action is now possible.