Sandwich itu saya bawa ke kantor untuk makan siang.

In Indonesian, words like ini and itu usually come after the noun, not before it.

So:

• sandwich itu = that sandwich
• buku ini = this book

In many contexts, itu can also make the noun sound definite, so sandwich itu may be understood as that sandwich or simply the sandwich, depending on context.

If you remove itu, sandwich sounds less specific:

• Saya bawa sandwich ke kantor = I brought a sandwich / brought sandwich(s) to the office

Starting with Sandwich itu puts focus or attention on the sandwich.

A more neutral order would be:

• Saya membawa sandwich itu ke kantor untuk makan siang.

But Sandwich itu saya bawa ke kantor... highlights the sandwich first, as if saying:

• That sandwich, I took it to the office for lunch.

This kind of fronting is very common in Indonesian.

This is a good question, because Indonesian works differently from English here.

Many grammars treat Sandwich itu saya bawa as a kind of passive-type construction often called passive type 2, where:

• the thing affected comes first: Sandwich itu
• the agent is a pronoun: saya
• the verb appears without meN-: bawa

So structurally, it is not the same as a normal active sentence like:

• Saya membawa sandwich itu ke kantor

However, in natural English, it is often translated actively:

• I brought that sandwich to the office for lunch

So the safest way to think of it is:

• grammatically, it is an object-first construction
• functionally, it often translates like a normal active sentence in English

Because after a fronted object and a pronoun like saya, Indonesian normally uses the bare verb:

• Sandwich itu saya bawa
• Buku itu saya baca
• Surat itu kami kirim

Using membawa here would sound wrong:

• Sandwich itu saya membawa ❌

If you want the ordinary active form, then membawa is fine:

• Saya membawa sandwich itu ke kantor

So:

• saya membawa ... = normal active
• ... saya bawa = object-first / passive-type pattern

It can mean either, depending on perspective.

In English, bring and take depend on viewpoint:

• bring = toward a place/viewpoint
• take = away to another place

Indonesian bawa and membawa are less strict about that distinction. They often just mean carry / bring / take.

So Sandwich itu saya bawa ke kantor could be translated as:

• I brought that sandwich to the office
• I took that sandwich to the office

Both are possible depending on the context and the English speaker’s viewpoint.

Ke is the preposition for movement toward a destination, so:

• ke kantor = to the office

Compare:

• di kantor = at the office
• ke kantor = to the office

Since the sandwich is being moved to the office, ke is the correct choice.

Untuk often expresses purpose, so untuk makan siang means something like:

• for lunch
• to have for lunch
• for eating at lunchtime

In this sentence, the natural idea is that the sandwich was brought to the office to be eaten as lunch or for lunchtime.

It does not necessarily spell everything out as explicitly as English sometimes does. Indonesian often leaves that kind of detail to context.

If someone wanted to be more explicit, they might say something like:

• Sandwich itu saya bawa ke kantor untuk dimakan saat makan siang.

That more clearly means:

• I took that sandwich to the office to eat at lunchtime.

Literally, makan siang is a verbal expression meaning eat lunch or have lunch.

• makan = eat
• siang = daytime / noon

But in Indonesian, verb phrases can often be used where English might use a noun phrase. So after untuk, makan siang works naturally to express the activity of having lunch.

That is why untuk makan siang sounds natural even though English often prefers something shorter like for lunch.

Yes. In everyday Indonesian, buat is very commonly used instead of untuk.

So you could say:

• Sandwich itu saya bawa ke kantor buat makan siang.

This is more casual and conversational.

Very roughly:

• untuk = more neutral or slightly more formal
• buat = more informal and spoken

Both are common and natural.

Yes. A more neutral active version is:

• Saya membawa sandwich itu ke kantor untuk makan siang.

A very natural spoken version is:

• Saya bawa sandwich itu ke kantor buat makan siang.

These all express roughly the same idea, but they differ in style:

• Sandwich itu saya bawa ke kantor...
  puts focus on the sandwich

• Saya membawa sandwich itu ke kantor...
  is more neutral and more formal

• Saya bawa sandwich itu ke kantor buat makan siang.
  is natural casual speech

So the original sentence is correct; it just has a particular focus and structure that English learners often notice.