gwaileun naengjanggo-e neoheo dumyeon orae gayo.

은 is the topic marker. In this sentence, 과일은 means something like as for fruit or when it comes to fruit.

That makes the sentence sound like a general statement about fruit in general:

• 과일은 냉장고에 넣어 두면 오래 가요.
• As for fruit, if you keep it in the fridge, it lasts a long time.

If you used 과일이, the focus would shift more toward fruit as the subject being identified or emphasized. Here, 은 is more natural because the speaker is giving a general fact or piece of advice.

It usually means fruit in general here.

Korean often does not mark singular vs. plural unless it needs to. So 과일 can mean:

• fruit
• a fruit
• fruits

In this sentence, it is best understood as fruit in general or fruit/fruits as a category.

Because 에 marks the destination/location that something is put into.

With 넣다 (to put in), the thing being placed takes 을/를, and the place it goes into often takes 에:

• 과일을 냉장고에 넣다
• to put fruit into the refrigerator

So in your sentence:

• 냉장고에 = into the refrigerator / in the refrigerator

Even though the object 과일을 is not repeated later in the sentence, the destination marker 에 still stays.

This is a very common Korean pattern:

• 넣다 = to put in
• 두다 = to leave something as it is, keep it that way
• -면 = if / when

So 넣어 두면 means:

• if you put it in and leave it there
• if you keep it stored inside
• if you put it away

The -아/어 두다 pattern often means doing something in advance and leaving it in that state.

Examples:

• 문을 열어 두다 = leave the door open
• 음식을 만들어 두다 = make food in advance and keep it ready
• 과일을 냉장고에 넣어 두다 = put fruit in the fridge and keep it there

넣으면 simply means if you put it in.

넣어 두면 adds the idea of putting it in and leaving it there / keeping it there.

So:

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

In this sentence, 넣어 두면 is more natural because fruit lasts longer not just from the moment you put it in, but from being stored there.

It can be understood as if or when, depending on context.

In general statements like this, -면 often means:

• if
• when
• whenever

So this sentence is giving a general truth or tip:

• If/When you keep fruit in the refrigerator, it lasts a long time.

It is not necessarily talking about one single future event. It is more like a general rule.

Yes, 가다 literally means to go, but Korean verbs often have extended meanings.

In the expression 오래 가다, it means:

• to last long
• to keep going for a long time
• to endure

So 오래 가요 here means:

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

This is a very common expression. Similar uses include:

• 이 신발은 오래 가요. = These shoes last a long time.
• 배터리가 오래 가요. = The battery lasts a long time.

Yes, the object is understood and omitted.

A more expanded version would be:

• 과일은 냉장고에 넣어 두면 오래 가요.
• understood as: 과일은 냉장고에 넣어 두면 과일이 오래 가요 or 과일은 냉장고에 넣어 두면 오래 가요

Korean often leaves out words that are obvious from context. Since 과일은 already introduced the topic, repeating 과일을 is unnecessary.

That omission is very natural in Korean.

Yes, you will often see both.

• 넣어 두면
• 넣어두면

The separated form makes the grammar easier to see:

• 넣어
  • 두면

In real-life writing, the joined form is also common. For a learner, it is helpful to recognize that both refer to the same structure: verb + 아/어 두다.

가요 is the polite, everyday style.

Here are the main versions:

• 가다 = dictionary form
• 가요 = polite, conversational
• 갑니다 = more formal
• 가 = casual/intimate

So this sentence is in a normal polite style, which is very common in speech and everyday writing:

• 과일은 냉장고에 넣어 두면 오래 가요.

A more formal version would be:

• 과일은 냉장고에 넣어 두면 오래 갑니다.

By itself, 오래 가요 simply means it lasts a long time.

But because of the context, English often interprets it as:

• it lasts longer
• it keeps longer

That comparison is implied by common sense: fruit kept in the fridge lasts longer than fruit not kept there.

Korean does not need to explicitly say 더 (more / longer) here for the idea to be understood.

It can sound like all three, depending on context.

Most naturally, it sounds like a general fact or practical advice:

• Fruit lasts a long time if you keep it in the fridge.

So a listener might understand it as:

• a statement of fact
• a tip
• a recommendation

That is very common with -면 sentences in Korean.