Penanda buku ini di dalam beg kertas.

A useful way to parse it is:

• penanda buku ini = this bookmark
• di dalam beg kertas = in / inside the paper bag

So the sentence structure is basically:

• subject/topic: penanda buku ini
• location phrase: di dalam beg kertas

Malay often puts things together very compactly, so once you spot the noun phrase and the location phrase, the sentence becomes much easier to read.

Because in Malay, demonstratives such as ini = this and itu = that usually come after the noun, not before it.

So:

• penanda buku ini = this bookmark
• beg kertas itu = that paper bag

This is normal Malay word order.
So even though English says this bookmark, Malay says the equivalent of bookmark this in word order, while still meaning this bookmark.

Malay usually does not need a present-tense verb like English is/are in this kind of sentence.

So English:

• This bookmark is in the paper bag.

Malay can simply say:

• Penanda buku ini di dalam beg kertas.

That is completely normal.

If you want, Malay can also use ada to emphasize existence or location:

• Penanda buku ini ada di dalam beg kertas.

But in many everyday sentences, leaving it out is natural and common.

di dalam means in or inside, with a clear sense of being physically inside something.

Here:

• di dalam beg kertas = inside the paper bag

A rough breakdown is:

• di = at / in / on
• dalam = inside / interior / in

Together, di dalam strongly suggests inside rather than just a general location.

Yes, in many contexts dalam by itself can also mean in or inside.

So these are both possible:

• di dalam beg kertas
• dalam beg kertas

The version with di dalam is a little more explicit about physical interior location.
For a learner, di dalam is a very safe and clear choice when you mean inside.

Because di here is a preposition meaning location, so it is written as a separate word:

• di dalam
• di rumah
• di sekolah

This is different from the passive prefix di-, which is attached directly to a verb:

• dibaca = read in the passive sense
• ditulis = written

So a very important spelling rule in Malay is:

• di as a place preposition → separate
• di- as a verb prefix → attached

Because Malay usually puts the main noun first and the describing noun after it.

So:

• beg kertas = literally bag paper
• natural English = paper bag

The head noun is beg = bag, and kertas tells you what kind of bag it is, or what it is made of.

This pattern is very common in Malay:

• rumah kayu = wooden house
• meja makan = dining table
• beg kertas = paper bag

By itself, it can be either. Malay does not normally use articles like a, an, and the.

So beg kertas could be understood in English as:

• a paper bag
• the paper bag

The right English choice depends on context.

That means when translating from Malay into English, you often have to add the article that sounds best in the situation, even though Malay did not say it explicitly.

No, not in this sentence.

Here, penanda buku ini is a noun phrase:

• penanda buku = bookmark
• ini = this

So it means this bookmark.

A command like bookmark this would need a verb, not just a noun phrase.
So even though the word order may look strange to an English speaker at first, penanda buku ini is not an imperative here.

Yes, penanda buku is a normal way to say bookmark.

Literally, it is something like:

• penanda = marker / indicator
• buku = book

So the whole expression is literally book marker, which matches the meaning of bookmark very well.

It is written as two words, not one.

Yes. beg is a Malay loanword from English bag.

That is why it looks familiar to an English speaker.
Malay has many borrowed words, and beg is one of the very common ones.

So in this sentence:

• beg = bag
• beg kertas = paper bag