geunyeoneun naengjanggo-e issneun soseureul kkeonaeseo saendeuwichie ballasseo.

는 is the topic marker. It tells you that 그녀 (she) is the topic of the sentence.

So 그녀는 means something like:

• As for her...
• She...

In many cases, this sounds natural in English as just she, but in Korean it also gives a slight sense of setting her up as the topic.

If you changed it to 그녀가, the feeling would be a bit different: 그녀가 would focus more directly on she as the subject, while 그녀는 is more like introducing or discussing her.

This is a very common question.

Here, 있는 comes from 있다, which means to exist / to be located / to be in.

With 있다, the place is marked with 에, not 에서.

So:

• 냉장고에 있다 = to be in the fridge

Then that whole idea becomes a noun-modifying phrase:

• 냉장고에 있는 소스 = the sauce that is in the fridge

Why not 에서?

• 에 is used for location/existence
• 에서 is used for where an action happens

Compare:

• 냉장고에 소스가 있어. = There is sauce in the fridge.
• 부엌에서 요리해. = I cook in the kitchen.

Since 있다 is not really an action like cook, 에 is the correct particle here.

있는 is the noun-modifying form of 있다.

Korean often puts a verb before a noun to describe that noun.

So:

• 있다 = to be / to exist
• 있는 = that is / which is

Therefore:

• 냉장고에 있는 소스 = the sauce that is in the fridge = more literally, the sauce existing in the fridge

This is a very common Korean pattern:

• 책상 위에 있는 책 = the book that is on the desk
• 학교에 다니는 학생 = the student who goes to school
• 어제 만난 사람 = the person I met yesterday

So 있는 is not a separate adjective here; it is a verb form used to modify 소스.

Because 소스 is the direct object of 꺼내다 (to take out / pull out).

So:

• 소스를 꺼내다 = to take out the sauce

The particle 을/를 marks the thing directly affected by the verb.

Here:

• 그녀는 냉장고에 있는 소스를 꺼내서...
• She took out the sauce that was in the fridge and...

So 소스 is what she took out, which is why it gets 를.

꺼내서 comes from:

• 꺼내다 = to take out
• -아서/어서 = a connector meaning and, then, or after doing

So:

• 꺼내서 = taking it out and...
• more naturally: took it out and...

In this sentence, it links two actions:

1. She took out the sauce.
2. She spread it on the sandwich.

So the sentence flows as:

• 그녀는 냉장고에 있는 소스를 꺼내서 샌드위치에 발랐어.
• She took out the sauce from the fridge and spread it on the sandwich.

A useful thing to know: in Korean, only the final verb usually carries the full tense/politeness information clearly, while earlier connected verbs often appear in a linking form like -서.

The object is understood from context: it is the sauce.

Korean often leaves out words that are obvious.

So even though the sentence literally says:

• She took out the sauce in the fridge and spread on the sandwich

the meaning is clearly:

• She took out the sauce in the fridge and spread the sauce on the sandwich

If you wanted to say it more explicitly, you could say:

• 그녀는 냉장고에 있는 소스를 꺼내서 샌드위치에 소스를 발랐어.

But that sounds a bit repetitive, so Korean often omits the repeated object.

With 바르다 (to spread/apply), Korean often marks the surface/target with 에.

So:

• 빵에 버터를 바르다 = to spread butter on bread
• 손에 크림을 바르다 = to apply cream to the hands
• 샌드위치에 소스를 바르다 = to spread sauce on the sandwich

In this pattern:

• the thing being spread/applied = object with 을/를
• the surface receiving it = 에

In your sentence, the object (소스) is omitted because it is obvious, but 샌드위치에 still shows where the sauce was spread.

발랐어 comes from 바르다 (to spread / apply) in the past casual form.

Breakdown:

• 바르다 = dictionary form
• 발랐어 = spread / applied

So this ending tells you:

• past tense
• casual/informal speech style

This is the kind of ending you would use with a friend, someone younger, or in informal narration.

If you wanted a polite version, you could say:

• 발랐어요

If you wanted a plain dictionary-style statement, you could say:

• 발랐다

This is normal in Korean.

In a chain of connected verbs, the final verb usually carries the tense for the whole sequence.

So:

• 꺼내서 발랐어

means:

• took it out and spread it

Even though 꺼내서 itself does not look explicitly past, the final 발랐어 makes the whole sequence understood as past.

This happens a lot in Korean:

• 집에 가서 잤어. = I went home and slept.
• 문을 열고 들어갔어. = I opened the door and went in.

The earlier action is interpreted in relation to the final tense.

Yes, grammatically it can be understood either way depending on context.

Korean does not have articles like a and the, so 소스 by itself does not tell you whether it is:

• a sauce
• the sauce

That is decided by context.

So 냉장고에 있는 소스 literally means:

• sauce that is in the fridge

In natural English, if both speaker and listener know which sauce is meant, it becomes:

• the sauce in the fridge

If the sauce is being introduced for the first time, it might be translated as:

• a sauce in the fridge

In many real situations, the sauce in the fridge is the most natural interpretation.

Yes, it is natural. But there are a few other ways a similar idea might be expressed depending on context.

For example:

• 샌드위치에 소스를 발랐어.
  Explicitly states the object.

• 샌드위치에 발라 먹었어.
  Adds the nuance of spreading it and eating it.

• 빵에 발랐어.
  If someone wants to refer specifically to the bread part.

Your sentence is natural because:

1. 소스 was already mentioned,
2. the omitted object is obvious,
3. 샌드위치에 바르다 is a normal pattern.

So the original sentence sounds smooth and ordinary in everyday Korean.