Jangan-jangan kostumnya disimpan di rumah.

Because jangan-jangan is a fixed idiomatic expression, not just jangan repeated literally.

• jangan by itself = don't
• jangan-jangan = maybe, what if, I have a suspicion that..., often with a worried or uneasy feeling

So in this sentence, it signals speculation, not a command.

It usually adds a sense of suspicion, concern, or nervous guessing.

It is often used when the speaker is thinking something like:

• Could it be that...
• What if...
• Don't tell me...

So the sentence does not sound like a neutral factual statement. It sounds more like the speaker is guessing and is somewhat uneasy or doubtful.

Because it is a reduplicated form. In Indonesian spelling, full reduplication is commonly written with a hyphen.

Examples:

• anak-anak = children
• pelan-pelan = slowly / take it easy
• jangan-jangan = perhaps / what if

In this case, the reduplication creates an idiomatic expression.

The suffix -nya can do several things in Indonesian, and here it most likely means one of these:

• his/her/their costume
• the costume already known from context

So kostumnya can be understood as:

• his costume
• her costume
• their costume
• or simply the costume

This is very common in Indonesian. -nya does not always have to point clearly to a specific person; sometimes it just makes the noun more definite.

Because this sentence is in the passive voice.

• simpan = store, keep
• disimpan = be stored, be kept

So kostumnya disimpan di rumah is literally structured like:

• the costume is kept/stored at home

Indonesian uses passive constructions very naturally, often more often than English does.

The sentence does not say.

That is normal in Indonesian passive sentences. The focus is on the costume, not on the person who stored it.

If the speaker wanted to mention the agent, they could add it, for example:

• Kostumnya disimpan oleh ibunya = The costume is kept by his/her mother.

But without that, the doer is simply left unspecified.

It can mean any of those, depending on context.

Indonesian verbs do not usually mark tense the way English verbs do. So disimpan by itself does not tell you whether the action is present, past, or a general state.

Context tells you the time:

• Sekarang jangan-jangan kostumnya disimpan di rumah = maybe the costume is being kept at home now
• Kemarin jangan-jangan kostumnya disimpan di rumah = maybe the costume was kept at home yesterday

Because these are two different di forms:

• di as a preposition = in / at / on → written separately
  
  • di rumah = at home / in the house
• di- as a passive verb prefix → written together
  
  • disimpan = stored / kept

This is a very common spelling point for learners.

So:

• di rumah ✔
• disimpan ✔

It can mean either, depending on context.

• di rumah literally = at home / in the house
• In natural English, you might choose one or the other depending on the situation

If the speaker means the costume might have been left where the person lives, at home is often the most natural translation.

Yes. Kostum is a normal Indonesian noun, borrowed from a foreign source but fully used in everyday Indonesian.

It usually means:

• costume
• outfit for a performance
• special clothing for an event

So kostumnya is completely natural Indonesian.

Yes, but the meaning and tone would change.

• Kostumnya disimpan di rumah.
  This sounds like a plain statement: The costume is kept at home.

• Jangan-jangan kostumnya disimpan di rumah.
  This sounds like a guess or suspicion: Maybe the costume is kept at home.

So jangan-jangan is important because it changes the sentence from a statement into a worried or tentative speculation.

This is a normal Indonesian passive pattern:

• subject/patient
  • passive verb
    • location

So:

• kostumnya = the costume
• disimpan = is kept / is stored
• di rumah = at home

This order is very natural in Indonesian. The sentence starts with the thing being talked about, then gives the passive verb, then the location.