geu sangja aneneun jageun bongtuga du gae deureo isseosseo.

그 literally means that.

So 그 상자 is literally that box. In real translation, though, it can sometimes sound more natural as the box if the box has already been mentioned or is obvious from context.

So depending on context:

• 그 상자 = that box
• sometimes naturally translated as the box

상자 안에는 breaks down like this:

• 상자 = box
• 안 = inside
• 에 = in, at
• 는 = topic marker, often with a contrastive nuance

So 상자 안에는 means something like:

• inside the box
• as for inside the box
• in the box, ...

The 는 can add a slight feeling of setting the scene or contrasting that location with others, as if saying inside the box, there were...

They are marking different things.

• 안에는: the location is marked as the topic
• 봉투가: the thing that exists in that location is marked with 가

This is a very common Korean pattern:

• location + 에는 + thing + 이/가 + 있다/없다

So here:

• 그 상자 안에는 = as for in that box
• 작은 봉투가 두 개 = two small envelopes
• 들어 있었어 = were inside

A very literal structure would be:

• As for inside that box, two small envelopes were in it

Because it is being used directly before a noun.

In Korean, descriptive words like 작다 change form when they modify a noun.

• 작다 = to be small
• 작은 = small, when placed before a noun

So:

• 작은 봉투 = a small envelope / small envelopes

This is similar to how English uses small before a noun, but in Korean the form changes.

Because native Korean numbers often change form before counters.

• 둘 = two
• before a counter, it becomes 두

And 개 is a very common counter for objects.

So:

• 두 개 = two items

That is why the sentence uses:

• 봉투가 두 개 = there were two envelopes

Even though 개 is a general counter, it is perfectly natural here.

Because the number already shows that it is plural.

Korean often does not use a separate plural marker when the amount is clear.

So:

• 봉투가 두 개 already clearly means two envelopes

Adding 들 would usually be unnecessary here.

English requires envelopes, but Korean does not need a special plural ending in this sentence.

들어 있었어 comes from 들어 있다, which means to be inside, to be contained in, or to be in a state of having gone inside.

So here it means:

• were inside
• were in there
• had been placed inside

This is a little different from just 있었어.

• 있었어 = existed / was there
• 들어 있었어 = was inside, was contained inside

It emphasizes being in something.

It is also different from 들어갔어:

• 들어갔어 = went in
• 들어 있었어 = was already in there

So this sentence is describing a state, not a movement.

Because it is based on the expression 들어 있다, which is often written as two parts.

• 들어 = having entered / in
• 있다 = to exist, to be

Together, they function like one expression meaning to be inside or to be contained.

For a learner, it is best to understand 들어 있다 as a set expression, even though it is often spaced as two words in writing.

Korean often places the count after the noun being counted.

So:

• 작은 봉투가 두 개 = there were two small envelopes

This is a very natural pattern, especially in existence-type sentences.

Another natural way to arrange it would be:

• 작은 봉투 두 개가 들어 있었어

Both are possible, but 봉투가 두 개 is very common when the sentence is basically saying there were two envelopes.

Yes. The ending -었어 is casual and informal.

So this sentence sounds like something you would say to a friend, someone younger, or in a relaxed context.

More polite versions would be:

• 그 상자 안에는 작은 봉투가 두 개 들어 있었어요.
• 그 상자 안에는 작은 봉투가 두 개 들어 있었습니다.

So the original sentence is correct, but it is in a casual style.