Buku itu ada di bagian atas lemari.

In Indonesian, demonstratives like ini (this) and itu (that) normally come after the noun.

• buku itu = that book / the book
• rumah ini = this house

So Buku itu is the normal Indonesian order.

Also, in many contexts, itu can feel like English the, not just that. So buku itu may mean either that book or the book, depending on context.

In this sentence, ada means something like is there, exists, or is located.

So:

• Buku itu ada di bagian atas lemari.
  = The book is located in/on the upper part of the cabinet.

When Indonesians talk about where something is, ada is very common. It helps show that the thing exists in that location.

Compare:

• Buku itu ada di meja. = The book is on the table.
• Ada buku di meja. = There is a book on the table.

So ada is often used for both there is/there are and is somewhere.

Indonesian often does not need a copula like English is/am/are.

For example:

• Dia guru. = She is a teacher.
• Rumahnya besar. = His/Her house is big.

But with location, Indonesian often uses ada instead of a plain is:

• Dia ada di rumah. = He/She is at home.
• Buku itu ada di sini. = The book is here.

So in this sentence, ada is doing the job that English often expresses with is in location sentences.

Break it down like this:

• di = in/at/on
• bagian = part, section
• atas = top, upper
• lemari = cupboard, cabinet, wardrobe

So di bagian atas lemari literally means:

• in the upper part of the cabinet
• at the top section of the cupboard
• depending on context, sometimes simply on top of the cabinet

The phrase is a little more specific than just di atas lemari.

• di atas lemari usually means on top of the cabinet
• di bagian atas lemari often suggests in the upper section/upper part of the cabinet

So this sentence may imply the book is in the top area or upper section, not necessarily sitting on the outside top surface.

Because lemari is part of the noun phrase bagian atas lemari.

This phrase works like:

• bagian atas lemari = the upper part of the cabinet

Here, lemari tells you whose upper part it is.

So the structure is:

• di
  • bagian atas lemari
• in
  • the upper part of the cabinet

Indonesian often shows this kind of relationship just by placing nouns together:

• pintu rumah = the door of the house
• warna mobil = the color of the car
• bagian atas lemari = the upper part of the cabinet

So a second di would not be correct here.

Yes, you could — but the meaning is slightly different.

• Buku itu di atas lemari.
  = The book is on top of the cabinet.

• Buku itu ada di bagian atas lemari.
  = The book is in the upper part / top section of the cabinet, or sometimes at the top part of the cabinet

So:

• use di atas lemari for something physically on top of the cabinet
• use di bagian atas lemari for the upper area/upper section of the cabinet

In real conversation, the exact meaning depends on context.

Lemari usually refers to a piece of furniture like a:

• cupboard
• cabinet
• wardrobe

The best English translation depends on context.

For example:

• in a bedroom, lemari may be a wardrobe
• in a kitchen or office, it may be a cabinet or cupboard

So if you already know what kind of furniture is being talked about, you choose the most natural English word from context.

Indonesian often expresses possession or of-relationships by placing the possessed thing first, followed by the possessor.

So:

• bagian atas lemari = the top part of the cabinet
• literally something like part upper cabinet

Other examples:

• buku guru = the teacher’s book / the book of the teacher
• atap rumah = the roof of the house
• kunci mobil = the car key

So lemari is not being described by bagian atas; rather, bagian atas belongs to or is part of the lemari.

Yes. In this sentence, di is a preposition meaning in, at, or on, so it is written separately:

• di bagian atas lemari
• di rumah
• di meja

This is different from the prefix di- used in passive verbs, which is written attached:

• ditulis = written
• dibaca = read
• ditutup = closed

So:

• di bagian = correct, because di is a preposition
• dibagian = incorrect here

Sometimes yes, especially in casual speech, but ada is the more standard and clearer choice for location.

Compare:

• Buku itu ada di bagian atas lemari.
  Very natural for saying where the book is.

• Buku itu di bagian atas lemari.
  Possible in conversation, but it may sound more shortened or less explicit.

So for learners, using ada in location sentences is a good habit.

Here it means top or upper.

Atas can appear in a few related meanings:

• di atas meja = on the table / literally above the table
• bagian atas lemari = the upper part of the cabinet
• atas by itself can mean top, upper part, or upstairs/above, depending on context

So in bagian atas lemari, it is clearly part of the noun phrase upper part of the cabinet, not just a simple preposition meaning above.