gwaileul naengjanggo-e neoheo dumyeon orae gayo.

Because 넣다 means to put something in, and the thing being put in is the direct object.
So 과일을 means fruit as the object of 넣다.

• 과일을 넣다 = to put fruit in
• 을/를 = object marker

If you changed it to 과일은, that would make fruit the topic, often with a contrastive feel: As for fruit, if you put it in the fridge...

에 is used here because it marks the destination or location something is placed into.

With verbs like 가다 (go), 오다 (come), 넣다 (put in), and 놓다 (place), 에 often marks where something ends up.

• 냉장고에 넣다 = put it into the refrigerator

에서 is more often used for the place where an action happens.

• 냉장고에서 먹어요 = eat in/from the refrigerator depending on context
• 냉장고에 넣어요 = put it in the refrigerator

So here, the important idea is the destination: into the fridge.

This is 넣다 + 두다.

• 넣다 = to put in
• -아/어 두다 = to do something and leave it that way, often for later convenience or as a maintained state

So 넣어 두다 means something like:

• put it in and leave it there
• store it there
• put it away for later

Then -면 adds if/when, so:

• 넣어 두면 = if/when you put it in and leave it there

This is more natural than just saying put it in because the sentence is about the fruit staying in the fridge and lasting longer.

You can say 넣으면, but it is slightly less specific.

• 넣으면 = if you put it in
• 넣어 두면 = if you put it in and keep it there

The version with 두다 emphasizes the resulting stored state. In this sentence, that matters, because fruit lasts longer not merely from the act of putting it in, but from being kept there.

So 넣어 두면 sounds more natural for storing food.

-면 means if or when, depending on context.

In this sentence, it introduces a condition:

• 과일을 냉장고에 넣어 두면 오래 가요.
• If/When you keep fruit in the refrigerator, it lasts a long time.

Korean often uses -면 for general truths too, not only one-time future situations.
So here it sounds like a general rule: If you refrigerate fruit, it keeps longer.

Yes, 가다 usually means to go, but it also has several extended meanings.
In the expression 오래 가다, it means:

• to last long
• to keep well
• to continue for a long time

So 오래 가요 here means:

• it lasts a long time
• it keeps for a long time

This is a very common expression in Korean.

Examples:

• 이 배터리는 오래 가요. = This battery lasts a long time.
• 이 꽃은 오래 가요. = These flowers last a long time.

오래 is an adverb meaning for a long time or long.

With 가다, the combination 오래 가다 is a set expression meaning:

• to last long
• to keep for a long time

So in this sentence:

• 오래 가요 = lasts long

This is more natural than translating word by word as goes long.
It is just one of those common Korean verb combinations you should learn as a chunk: 오래 가다.

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

In this sentence, the general meaning is clear even without saying a subject like it or fruit again. Korean commonly leaves out words that English would normally include.

A more expanded English-style understanding would be:

• If you put fruit in the refrigerator, it lasts longer.

But Korean does not need to explicitly say it.

This kind of omission is very normal and natural in Korean.

It can mean either, depending on context.

Korean nouns usually do not have to show singular vs. plural the way English nouns do. So 과일 can mean:

• fruit in general
• some fruit
• fruits

In this sentence, it most naturally means fruit in general.

That is why English translations may vary:

• Fruit lasts longer...
• Fruits last longer...
• If you keep fruit in the fridge...

All can fit, depending on how natural the translator wants the English to sound.

가요 is in the informal polite style, often called 해요체.

It is polite and very common in everyday conversation.

Here are a few versions of the same ending:

• 가다 = dictionary form
• 가요 = polite everyday style
• 갑니다 = more formal polite style
• 가 = casual style

So the sentence is phrased in a normal, friendly polite way.

Yes:

• 과일을 = fruit + object marker
• 냉장고에 = in/into the refrigerator
• 넣어 두면 = if you put it in and leave it there
• 오래 가요 = it lasts a long time

A very literal gloss would be:

Fruit, into the refrigerator, if you put it and leave it there, it lasts long.

That is not natural English, but it shows how the Korean sentence is built.

It most naturally sounds like a general fact or habitual truth.

Because of -면, the sentence can mean something like:

• If you refrigerate fruit, it lasts longer.
• When fruit is kept in the fridge, it keeps longer.

So it sounds like practical advice or a general observation, not just a one-time event.

If someone wanted to talk about one specific past case, they would use a different form.