piltong ane bolpeni se gaena deureo isseo.

안 means inside, and 에 marks a location.

So:

• 필통 = pencil case
• 안 = inside
• 에 = at / in

Together, 필통 안에 means inside the pencil case or in the pencil case.

Korean often builds location phrases this way:

• 상자 안에 = in the box
• 가방 안에 = in the bag
• 주머니 안에 = in the pocket

Use 에 when something exists or is located somewhere.

In this sentence, the pens are simply in the pencil case, so 에 is correct.

• 필통 안에 볼펜이 있어. = There is a pen in the pencil case.

에서 is usually used for:

• the place where an action happens
• the starting point of movement in some cases

For example:

• 학교에서 공부해. = I study at school.
• 집에서 먹어. = I eat at home.

Since this sentence is about where something is, not where an action is happening, 에 is the natural choice.

Here, 볼펜이 is the thing that exists in the pencil case, so it takes the subject marker 이/가.

In existence/location sentences, Korean often uses this pattern:

[place]에 [thing]이/가 있다

• 책상 위에 책이 있어.
• 방에 사람이 있어.

So:

• 필통 안에 = in the pencil case
• 볼펜이 = the ballpoint pen(s)
• 들어 있어 = are inside / are contained

Using 이/가 is very common in this kind of sentence because it highlights the item that is present.

Korean uses native Korean numbers with most general counters, including 개.

So for counting objects, you say:

• 한 개 = one
• 두 개 = two
• 세 개 = three
• 네 개 = four

Not:

• 일 개
• 이 개
• 삼 개

So 세 개 is the correct form for three items.

개 is a counter for counting general objects.

In Korean, you usually cannot just say a number directly before a noun when counting. You need a counter.

So:

• 볼펜 세 개 = three pens
• literally: ballpoint pens three units

Some common counters are:

• 개 for general things
• 명 for people
• 권 for books
• 잔 for cups/glasses of drinks

Since pens are ordinary objects, 개 works here.

The 나 adds emphasis. It means something like:

• as many as three
• no less than three
• three, surprisingly

So 세 개나 is not just a neutral three. It suggests the speaker feels that three is more than expected, a lot, or somehow notable.

Compare:

• 볼펜이 세 개 들어 있어. = There are three pens inside.
• 볼펜이 세 개나 들어 있어. = There are as many as three pens inside.

The exact feeling depends on context, but 나 usually adds surprise, emphasis, or a sense that the amount is significant.

Both can work, but 들어 있어 is more vivid and specific.

• 있어 = exists / is there
• 들어 있어 = is inside / is contained / is in there

So:

• 필통 안에 볼펜이 세 개 있어. = There are three pens in the pencil case.
• 필통 안에 볼펜이 세 개나 들어 있어. = There are as many as three pens inside the pencil case.

들어 있어 emphasizes that the pens are inside the container. It gives a stronger sense of being contained in it.

들어 있어 is the informal spoken form of 들어 있다.

들어 있다 means something like:

• to be in
• to be contained in
• to be inside

It describes a state: something has gone in and is now staying there.

So 볼펜이 세 개나 들어 있어 means:

• Three pens are inside
• The pencil case contains as many as three pens

In casual speech, -아/어 있다 often becomes -아/어 있어:

• 앉아 있다 → 앉아 있어
• 서 있다 → 서 있어
• 들어 있다 → 들어 있어

Korean often leaves plurality unmarked when the number is already clear.

Since 세 개 already tells us there are three pens, there is no need to add 들.

So:

• 볼펜이 세 개 = three pens
• not usually 볼펜들이 세 개

Adding 들 here would sound unnecessary or unnatural in most contexts.

Korean uses 들 only when it wants to emphasize plurality, and even then it is much less common than in English.

Yes, Korean word order is somewhat flexible, especially for phrases like location and quantity.

The given sentence is natural:

• 필통 안에 볼펜이 세 개나 들어 있어.

You could also hear:

• 볼펜이 필통 안에 세 개나 들어 있어.
• 필통 안에 세 개나 들어 있는 볼펜이 있어. (different structure, more like there are pens, as many as three, inside)

But the original order is very standard because it goes from:

1. location
2. thing
3. quantity
4. verb

That makes it easy to process.

Grammatically, the thing marked by 이 is 볼펜, so the sentence is saying that the pens are inside.

But in English, we often translate it in a way that focuses on the container:

• There are as many as three pens in the pencil case.
• The pencil case contains as many as three pens.

So Korean is structured around the pens being in there, while English may focus either on the pens or on the pencil case.

들어 있어 is casual/informal speech.

The more neutral dictionary-style form is:

• 들어 있다

A polite version would be:

• 필통 안에 볼펜이 세 개나 들어 있어요.

So the levels are roughly:

• 들어 있다 = plain/base form
• 들어 있어 = casual spoken
• 들어 있어요 = polite spoken

Because it is made from:

• 들어
  • 있어

This comes from the grammar pattern -아/어 있다, which combines a verb stem with 있다 to describe a continuing state.

So the spacing reflects that structure:

• 앉아 있어
• 놓여 있어
• 들어 있어

In actual speech, it sounds closely connected, but standard spacing keeps them separate.

Yes. The strongest clue is 나 in 세 개나.

Without 나, the sentence would sound more neutral:

• 필통 안에 볼펜이 세 개 들어 있어. = There are three pens in the pencil case.

With 나, it can imply:

• Wow, there are three pens in there
• There are as many as three
• That’s more than I expected

So if you want the nuance, think of 세 개나 as as many as three rather than just three.