Learn Hardcore Indonesian: Jumlah buku di perpustakaan itu sangat banyak. - The number of books in that library is very large.

Jumlah literally means “number / total / amount”. It points to the total count of something.
Banyak means “many / a lot”, describing how large that number or amount is.

In the sentence:

Jumlah buku = the number of books
sangat banyak = is very large / is very many

So the structure is basically:
“The number of books in that library is very large.”

You could also express a similar idea without jumlah, but then the grammar changes (see below).

Is it redundant to say jumlah … sangat banyak? Could I just say buku di perpustakaan itu sangat banyak?

It’s a bit redundant in a logical sense (you’re saying “the number is very many”), but it’s natural Indonesian, especially in more formal or careful speech/writing.

Jumlah buku di perpustakaan itu sangat banyak.
Emphasizes the total number of books is very big. Slightly more formal, more “report-like”.
Buku di perpustakaan itu sangat banyak.
Literally “The books in that library are very many.”
This is also natural, a bit more direct and slightly less formal.

Both are correct. The jumlah version sounds more like you’re talking about statistics, figures, or capacity. The version without jumlah sounds more like a general observation.

Why is itu placed after perpustakaan instead of before, as in “that library”?

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

perpustakaan itu = that library
perpustakaan ini = this library
orang itu = that person
rumah ini = this house

So:

English: that library
Indonesian: perpustakaan itu

Putting itu before the noun (e.g. itu perpustakaan) is possible but has a different feel: it usually works more like “that one, the library” or you’re pointing out something already mentioned. For a simple noun phrase “that library,” perpustakaan itu is the normal pattern.

What is the role of di in di perpustakaan itu, and how is it different from pada or di dalam?

di is the basic preposition meaning “in / at / on” (location).

di perpustakaan itu = in/at that library

Comparisons:

di perpustakaan itu
Neutral and most common for location: in/at that library.
di dalam perpustakaan itu
Literally inside that library. Used if you really want to stress “inside (the interior of) the building”, but often di perpustakaan itu alone is enough.
pada
More formal and less common for physical location in modern Indonesian. You’d more often see pada with time or abstract things:
- pada hari Senin = on Monday
- pengaruhnya pada masyarakat = its effect on society

So in everyday Indonesian, di perpustakaan itu is the natural choice for “in that library.”

How can I tell if buku is singular or plural? Why isn’t it buku-buku?

Indonesian usually doesn’t mark plural on nouns. Buku can mean “book” or “books”, depending on context.

In this sentence:

Jumlah buku di perpustakaan itu sangat banyak.

Because you’re clearly talking about a large number, it must be plural: books. The context (“the number is very many”) forces the plural meaning.

What about buku-buku?

buku-buku literally signals plural (“books”), but
Indonesian doesn’t need plural marking most of the time, and jumlah buku-buku sounds awkward and redundant here.

More natural:

jumlah buku (not jumlah buku-buku)
banyak buku (not banyak buku-buku)

Where does sangat go in the sentence, and how is it different from sekali?

Sangat means “very” and normally comes before an adjective:

sangat banyak = very many / very much
sangat besar = very big
sangat mahal = very expensive

Sekali also means “very”, but it goes after the adjective:

banyak sekali = very many
besar sekali = very big
mahal sekali = very expensive

So you can say:

Jumlah buku di perpustakaan itu sangat banyak.
Jumlah buku di perpustakaan itu banyak sekali.

Both sound natural. Differences:

sangat: a bit more formal/neutral, often used in writing.
sekali: sounds a bit more colloquial/natural in everyday speech.

You don’t combine them like sangat banyak sekali – that’s usually considered too much or incorrect in standard Indonesian.

What is the grammatical structure of the sentence? Which part is the subject and which is the predicate?

The sentence is:

Jumlah buku di perpustakaan itu = subject (noun phrase)
“The number of books in that library”
sangat banyak = predicate (adjective phrase)
“is very many / is very large (in number)”

Indonesian doesn’t need a separate verb “to be” (like is/are) in this kind of sentence. So:

English: The number of books in that library *is very large.*
Indonesian: Jumlah buku di perpustakaan itu sangat banyak.
(no explicit “is”)

Could I say Ada banyak buku di perpustakaan itu instead? What’s the difference in nuance?

Yes, Ada banyak buku di perpustakaan itu is very natural and means roughly:

There are many books in that library.

Comparison:

Jumlah buku di perpustakaan itu sangat banyak.
- Focuses on the total number.
- Feels a bit more formal, more like reporting a fact or statistic.
Ada banyak buku di perpustakaan itu.
- Focuses on the existence: “There are many books there.”
- Very common in everyday speech.
- Structure: ada = there is/are.

Both are correct; choose based on whether you want to highlight the number (jumlah) or simply the existence (ada) of many books.

Is this sentence formal, informal, or neutral? In what contexts would it sound natural?

Jumlah buku di perpustakaan itu sangat banyak. sounds neutral to formal.

It fits well in:

A report or article about a library
A school presentation
Written descriptions or official documents
News or informational programs

In casual conversation, people might more often say:

Ada banyak buku di perpustakaan itu.
Buku di perpustakaan itu banyak banget. (very colloquial, using banget = very)

So the given sentence is completely natural, just slightly more careful / formal than everyday speech.

Can I move the phrase di perpustakaan itu to another position, like Di perpustakaan itu jumlah buku sangat banyak?

Yes, Indonesian word order is quite flexible with location phrases like di perpustakaan itu.

These are all grammatical:

Jumlah buku di perpustakaan itu sangat banyak.
Di perpustakaan itu jumlah buku sangat banyak.

Nuance:

Version 1 is the most neutral and common.
Version 2 slightly emphasizes the place: In that library, the number of books is very large. (Focusing first on “in that library”.)

Both are fine; the first is more typical as a default sentence.