Learn Hardcore Indonesian: Hari ini saya sangat produktif di perpustakaan. - Today I am very productive at the library.

saya sangat produktif literally = I very productive
In English we must say I am very productive, but in Indonesian the am is simply omitted.

So:

Saya sangat produktif. = I am very productive.
Dia guru. = He/She is a teacher.
Mereka senang. = They are happy.

You add a form of to be only in special structures (e.g. adalah in some formal sentences), but not in a basic sentence like this one.

How do we know if this means “I am very productive” or “I was very productive”?

Indonesian verbs generally don’t change form for tense. Produktif is an adjective, so tense is understood from time words and context.

Hari ini = today → can refer to earlier today (past) or today in general (present).
Context decides whether you mean I was very productive (earlier today) or I am being very productive (now, today).

If you want to be more precise:

Tadi saya sangat produktif di perpustakaan.
= Earlier today I was very productive at the library.
Sekarang saya sangat produktif di perpustakaan.
= Right now I am very productive at the library.
Besok saya akan sangat produktif di perpustakaan.
= Tomorrow I will be very productive at the library.

What exactly does hari ini mean, literally and in use?

Literally, hari ini is:

hari = day
ini = this

So hari ini = this day, which corresponds to today.

Usage:

Hari ini panas. = Today is hot.
Hari ini saya sibuk. = I’m busy today.

You can also put hari ini later in the sentence:

Saya sangat produktif di perpustakaan hari ini.
Same meaning, just different emphasis: more focus on I was productive at the library, and then you add today at the end.

What is the difference between saya and aku? Could we say Hari ini aku sangat produktif di perpustakaan?

Yes, you can say:

Hari ini aku sangat produktif di perpustakaan.

The main difference:

saya = polite, neutral, slightly formal; safe in most situations, including with strangers, work, or older people.
aku = more informal, used with friends, family, or people of similar age or status.

Same meaning (I), different level of formality.

Examples:

To your boss: Hari ini saya sangat produktif.
To a close friend: Hari ini aku sangat produktif.

What does sangat do, and can it be placed somewhere else?

Sangat is an intensifier: it means very.

produktif = productive
sangat produktif = very productive

Typical position:

sangat comes before the adjective:
sangat produktif, sangat cepat, sangat cantik

You can’t normally move sangat after the adjective. Instead, Indonesian has another intensifier that comes after:

produktif sekali = very productive
cantik sekali = very beautiful

So:

sangat produktif and produktif sekali both mean very productive, just with different word order and slightly different flavor (see next question).

Is there any difference between sangat produktif and produktif sekali?

Both mean very productive, but there is a slight nuance:

sangat produktif
- More neutral and slightly more formal.
- Common in writing and polite speech.
produktif sekali
- Also standard Indonesian, but in everyday speech it can feel a bit more expressive or “colloquial-flavored,” depending on tone.
- sekali literally means once, but in this position it functions like very or really.

In casual speech with friends, people often use banget instead:

Hari ini saya produktif banget di perpustakaan.
(colloquial: I was really productive at the library today.)

What does di mean, and why is it di perpustakaan, not something else?

Di is a preposition meaning at / in / on (location, not movement).

di rumah = at home / in the house
di sekolah = at school
di perpustakaan = at the library / in the library

You use di when you’re talking about where something is happening.

If you talk about movement towards a place, you usually use ke:

Saya pergi ke perpustakaan. = I go / went to the library.
Saya belajar di perpustakaan. = I study at/in the library.

Why is there no word for “the” in di perpustakaan?

Indonesian does not have articles like a, an, or the. The noun is bare:

perpustakaan can mean a library or the library, depending on context.

So:

di perpustakaan can be:
- in a library
- in the library
- at the library

If you really want to specify the in a more definite sense, you can use:

perpustakaan itu = that library / the library (already known from context)
Hari ini saya sangat produktif di perpustakaan itu.
= Today I was very productive at that (particular) library.

Can di be written together with the noun, like diperpustakaan?

Not in this case. For location, di must be separate:

Correct: di perpustakaan
Incorrect: diperpustakaan

When di is written attached to a verb (like dimakan, dibaca), it’s no longer a preposition but a passive voice prefix.

baca = read
dibaca = is read / was read
makan = eat
dimakan = is eaten / was eaten

But with nouns of place, keep di separate:

di rumah, di kantor, di perpustakaan

What does perpustakaan literally mean, and is it always “library”?

Perpustakaan means library.

Morphologically, it’s:

pustaka = book or literature (a bit formal/old-fashioned)
per- ... -an = a common Indonesian noun-forming pattern, often for places related to something

So perpustakaan is literally like “place of books” → library.

In everyday speech:

perpustakaan = library (any library: school library, city library, etc.)

Colloquially, people also shorten it:

perpus (slang/short form)
Hari ini gue produktif banget di perpus.
= casual, to friends.

Is the word order here fixed, or can we say Saya sangat produktif di perpustakaan hari ini?

You can change the word order a bit, and it’s still natural:

Hari ini saya sangat produktif di perpustakaan.
- Time (hari ini) is emphasized at the beginning.
Saya sangat produktif di perpustakaan hari ini.
- Focus is more on I was very productive at the library, and hari ini is like an extra time detail.
Saya sangat produktif hari ini di perpustakaan.
- Also understandable/natural, though slightly less smooth than 1 and 2.

General pattern: time expressions (hari ini, kemarin, besok) can appear at the beginning or end of the sentence. Both are common.

Could we leave out saya and just say Hari ini sangat produktif di perpustakaan?

Grammatically, Indonesian often allows dropping pronouns if the subject is clear from context, but:

Hari ini sangat produktif di perpustakaan.
feels incomplete/awkward on its own, because who is productive is not obvious from the sentence.

In natural conversation, you might drop saya only if:

It’s very clear from the surrounding context that you are talking about yourself, and
The rest of the conversation already established the subject.

But as a stand-alone sentence or in careful speech, you should keep saya:

Hari ini saya sangat produktif di perpustakaan.

Is produktif just a borrowed word from English productive?

Yes. Produktif comes from the same Latin root and is a cognate with English productive.

Meaning and usage are very similar:

Dia sangat produktif. = He/She is very productive.
Tahun ini perusahaan kami kurang produktif.
= This year our company is less productive.

It’s used for people, companies, activities, etc., just like in English.

You could also express the idea with other words:

saya banyak menyelesaikan tugas = I finished many tasks
hari ini saya rajin belajar = today I studied diligently
but sangat produktif is perfectly natural and common.

How would this sentence sound in a very casual, slangy style among young friends?

A typical casual version might be:

Hari ini gue produktif banget di perpus.

Changes:

saya → gue (very informal “I” in Jakarta slang)
sangat → banget (very informal “very/really”)
perpustakaan → perpus (shortened slang for “library”)

Same basic structure and meaning, just with informal vocabulary.