Learn Hardcore Indonesian: Menurut saya, perpustakaan itu terbaik untuk belajar. - In my opinion, that library is the best for studying.

Correct: Perpustakaan itu terbaik untuk belajar.
Also correct (more formal/equational): Perpustakaan itu adalah yang terbaik untuk belajar or Perpustakaan itu adalah tempat terbaik untuk belajar.
Incorrect: Perpustakaan itu adalah terbaik… (don’t use adalah directly before a bare adjective)

Should I add yang before terbaik?

You don’t have to, but it changes the feel slightly.

… itu terbaik …: perfectly fine; reads like “is best.”
… itu yang terbaik …: emphasizes selection/contrast, “is the best one.” It sounds a bit more explicit and common in careful speech/writing.

Is the original sentence natural? What’s a very idiomatic version?

It’s natural, but many speakers would say one of these:

Menurut saya, perpustakaan itu yang terbaik untuk belajar.
Menurut saya, perpustakaan itu tempat terbaik untuk belajar. (very idiomatic)
Informal: Menurut saya, perpustakaan itu paling cocok/nyaman buat belajar.

What exactly does itu do here?

Itu marks definiteness (“that”/“the”).

perpustakaan itu = that specific library / the (aforementioned) library
perpustakaan ini = this library (near the speaker)
perpustakaan tersebut = that said/aforementioned library (formal, written tone) Without itu/ini/tersebut, perpustakaan could mean “a/the library” in general.

Where is the verb “is”? Why can an adjective stand alone?

In Indonesian, adjectives can function as predicates without a linking verb. So terbaik serves as the predicate: “(is) best.” No extra word like “is” is needed.

Can I move Menurut saya to another position?

Yes. All of these are fine:

Menurut saya, perpustakaan itu…
Perpustakaan itu, menurut saya, …
Perpustakaan itu …, menurut saya. Spoken Indonesian often places it at the start or end. Use a comma in writing to mark the pause.

What’s the difference between Menurut saya, saya pikir, and saya rasa?

Menurut saya = “in my opinion” (neutral/formal-ish; very common)
Saya pikir = “I think” (cognitive/judgment; neutral)
Saya rasa = “I feel/I think” (slightly more subjective/intuitive) All are acceptable; menurut saya feels most neutral and common in careful speech.

Why is it untuk belajar and not something else? Can I use buat?

Untuk marks purpose: “for/to (the purpose of) studying.” In everyday speech, buat is a very common informal substitute.

Formal/neutral: untuk belajar
Informal: buat belajar Other options: guna belajar (formal/literary), bagi is not used for purpose like this.

Should it be belajar or mempelajari?

Use belajar. It’s intransitive: “to study/learn.”

belajar = to study (in general)
mempelajari = to study/examine something (needs an object): mempelajari biologi

Why don’t we say di perpustakaan here?

Because perpustakaan itu is the subject (“that library”). If you say di perpustakaan, it becomes a location phrase (“at the library”). Different structures:

Subject: Perpustakaan itu terbaik untuk belajar.
Location-led: Di perpustakaan itu paling enak belajar. (Informal, “Studying is nicest at that library.”)

Is perpustakaan singular or plural here?

With itu, it refers to one specific library (singular). Indonesian doesn’t mark plural on the noun; context or doubling can show plurality:

perpustakaan-perpustakaan = libraries (formal/written emphasis on plural)

What’s the difference between terbaik and paling baik?

They both mean “best.”

terbaik = lexicalized superlative, very common and concise
paling baik = “most good,” equally correct and natural For some adjectives, ter- isn’t used; then paling + adj is the go-to superlative. With baik, both work.

When should I add tempat (“place”)?

Add tempat if you want to say “the best place to study,” which is very idiomatic:

Perpustakaan itu tempat terbaik untuk belajar. Without tempat, you’re saying “That library is (the) best for studying,” which is also fine.

Can I make this more informal/natural in conversation?

Yes. Common tweaks:

Use buat: … paling enak buat belajar.
Use opinion softeners: Menurut saya sih, …
Use paling nyaman/condusif/cocok instead of terbaik: … paling nyaman buat belajar.

Could I say perpustakaan ini instead of itu?

Yes, if you mean “this library” (near you or the one you’re in). Use ini for proximity/current focus; itu for something less immediate or already mentioned.

Is the comma after Menurut saya required?

In writing, yes, it’s standard to put a comma after an initial discourse marker like Menurut saya, because there’s a pause. In speech, it’s just a natural pause.