aiga mollae bange deureogassseupnida.

이/가 marks the subject, while 은/는 marks the topic.

In 아이가 몰래 방에 들어갔습니다, 아이가 simply identifies the child as the one who did the action.

If you said 아이는 몰래 방에 들어갔습니다, it would sound more like:

• As for the child, he/she secretly went into the room
• or it might suggest contrast, such as the child did that, unlike someone else

So 아이가 is the more neutral choice here.

몰래 is an adverb. It means something like:

• secretly
• stealthily
• without others knowing

It describes how the child entered the room.

You can think of it as modifying the verb 들어갔습니다:

• 몰래 들어갔습니다 = went in secretly

A related pattern you may also see is:

• N 몰래 = secretly, without N knowing
  Example: 엄마 몰래 = without Mom knowing / secretly from Mom

But in your sentence, 몰래 is just being used on its own as an adverb.

에 is used here to mark the destination of movement.

So:

• 방 = room
• 방에 = into/to the room

With motion verbs such as 가다 (to go), 오다 (to come), and 들어가다 (to enter/go in), 에 often marks the place someone moves to.

In this sentence:

• 방에 들어갔습니다 = went into the room

들어가다 means to go in / to enter. It is more specific than 가다, which just means to go.

So:

• 갔습니다 = went
• 들어갔습니다 = went in / entered

Because the sentence is about going into a room, 들어가다 is the natural verb.

It comes from the verb 들어가다.

Breakdown:

• 들어가다 = to go in, to enter
• past tense: 들어갔다
• formal polite ending: 들어갔습니다

So 들어갔습니다 means:

• entered
• went in

This is the past tense + formal polite style.

-습니다 is the formal polite sentence ending.

It is commonly used in:

• news
• presentations
• announcements
• formal writing
• polite speech to people you should speak respectfully to

The less formal polite version would be:

• 아이가 몰래 방에 들어갔어요

Both are polite, but -습니다 sounds more formal and stiff than -어요.

Sometimes yes, but 방에 is more natural here.

Both 에 and 으로 can relate to movement, but they are not always identical.

• 방에 들어갔습니다 focuses on the destination: entered the room
• 방으로 often emphasizes direction: toward the room

Since 들어가다 already includes the idea of going inward, 방에 들어갔습니다 is the standard, natural expression.

Korean word order is flexible compared with English, especially for adverbs.

So these can all be possible depending on context:

• 아이가 몰래 방에 들어갔습니다
• 아이가 방에 몰래 들어갔습니다
• 몰래 아이가 방에 들어갔습니다

The first one is very natural and straightforward.

Even though the order can move around, the particles help show each word’s role:

• 아이가 = subject
• 방에 = destination

So the meaning usually stays clear.

No. 아이 is gender-neutral.

It simply means:

• child
• kid

So from this sentence alone, you cannot tell whether the child is a boy or a girl.

Yes. 몰래 adds the idea that the action was hidden or done without being noticed.

So the sentence does not just say the child entered the room. It suggests something like:

• the child sneaked into the room
• the child went into the room secretly

That nuance is important because it tells you about the manner of the action, not just the action itself.