Learn Hardcore Indonesian: Saya menulis kenangan penting di buku harian supaya tidak lupa. - I write important memories in my diary so I don’t forget.

saya: neutral–polite, safe in most situations (talking to strangers, teachers, in writing, etc.).
aku: informal, used with close friends, family, or when talking to someone younger.

So:

Saya menulis kenangan penting di buku harian supaya tidak lupa. – polite/neutral.
Aku menulis kenangan penting di buku harian supaya tidak lupa. – sounds more casual/intimate.

Grammatically, both are correct; choose based on how close you are to your listener and how formal you want to sound.

Why is it menulis, not menuliskan? Are menulis and menuliskan different?

Both come from the root tulis (write).

menulis = to write (neutral, the most common form).
menuliskan = to write something for someone or to write something down, often with a bit more emphasis on the result or on doing it for someone.

Examples:

Saya menulis kenangan penting. – I write important memories. (neutral)
Saya menuliskan kenangan penting untuk anak-anak saya. – I write important memories for my children.

In your sentence, menulis is perfectly natural and most common. Menuliskan would not be wrong, but it sounds slightly heavier/more formal and often implies a more purposeful “writing down (for a reason/for someone)” feeling.

What exactly does kenangan mean? Is it just “memories”?

Kenangan comes from the root kenang (to remember, to recollect).

kenangan = a memory / recollection, usually with some emotional weight, often about past experiences.

So kenangan penting is important memories (significant experiences you remember).

Related forms you might see:

mengenang = to remember (often something/someone who has passed, or past events)
kenang-kenangan = a keepsake / souvenir (literally “something for remembering”)

Why is it kenangan penting, not penting kenangan? Where do adjectives go?

In Indonesian, adjectives almost always come after the noun:

kenangan penting = important memories
buku besar = big book
rumah baru = new house

So penting kenangan is wrong in this structure. The pattern is generally:

noun + adjective
kenangan penting, buku harian, film bagus, etc.

What does buku harian literally mean? Is it the same as “diary”?

Yes, buku harian is the usual word for a diary or journal.

buku = book
harian = daily / relating to daily life

So literally, buku harian is a daily book, i.e., a book you write in regularly about your life.

A couple of related terms:

diari – a borrowing from English diary, used but less common than buku harian in many contexts.
agenda – can mean a planner or schedule book, usually for appointments, not for personal feelings/memories.

In your sentence, buku harian is natural for a personal diary.

Why is di used in di buku harian? Can I say dalam buku harian instead?

di and dalam both relate to location, but they’re not the same.

di = at/in/on (general location or place). Very common, often short and neutral.
dalam = inside / within (more explicitly “inside something”).

In writing contexts:

di buku harian – literally “in the diary”, very common and natural.
di dalam buku harian – more explicit: “inside the diary”, a bit more emphatic or formal.
dalam buku harian – also possible, but di buku harian is more typical, short, and idiomatic.

So yes, dalam buku harian or di dalam buku harian can work, but di buku harian is the most natural everyday choice here.

Could I use pada instead of di here, like pada buku harian?

In this sentence, di is much more natural than pada.

di is used for physical location: di rumah, di meja, di buku harian.
pada is more abstract and is often used:
- with time expressions: pada hari Senin, pada tahun 2020
- with more formal or abstract objects: pada kesempatan ini, pada kalimat ini (in this sentence)

Pada buku harian is not wrong grammatically, but it sounds odd/unnatural in everyday Indonesian. Stick to di buku harian here.

What does supaya mean here? How is it different from agar and untuk?

In this sentence:

supaya tidak lupa

means so (that I) don’t forget.

Comparisons:

supaya – so that / in order that (very common, neutral).
agar – also so that / in order that, usually slightly more formal or written.
untuk – for / to (to do something), usually followed by a verb or noun (purpose).

You could say:

Saya menulis… supaya tidak lupa. – natural, neutral.
Saya menulis… agar tidak lupa. – also correct, a bit more formal.
Saya menulis… untuk tidak lupa. – possible, but supaya/agar sound more natural in this specific “so that I don’t forget” meaning.

When the idea is “so that (something will happen / not happen)”, supaya and agar are usually preferred.

Why is there no subject after supaya? Shouldn’t it be supaya saya tidak lupa?

Both are possible:

supaya tidak lupa
supaya saya tidak lupa

Indonesian often omits the subject if it’s clear from context. Since the sentence starts with Saya menulis…, it’s understood that saya is also the subject of tidak lupa.

Adding saya:

Saya menulis kenangan penting di buku harian supaya saya tidak lupa.

This is still correct and clear, but slightly longer and more explicit. Many native speakers would naturally drop the second saya here because it’s already obvious.

Why is it tidak lupa and not something like tidak melupakan or supaya ingat?

All of these are possible, but they sound slightly different:

tidak lupa = not forget (focus on avoiding forgetting)
melupakan = to forget something (transitive, usually needs an object: melupakan janji – forget a promise)
ingat = to remember

Your sentence:

supaya tidak lupa – aims at preventing forgetting in general (no specific object stated).

Alternatives:

supaya ingat – so that (I) remember (more positive framing).
supaya tidak melupakan hal-hal penting – so that (I) don’t forget important things (more specific, with an object).

Supaya tidak lupa is very natural and idiomatic when talking about writing things down to avoid forgetting them.

Why is the sentence Saya menulis kenangan penting di buku harian… and not Saya menulis di buku harian kenangan penting…?

Indonesian prefers a more “logical” order: verb + object + place.

Saya menulis kenangan penting di buku harian…
- menulis (verb)
- kenangan penting (what you write – object)
- di buku harian (where you write – place)
Saya menulis di buku harian kenangan penting…
- This becomes confusing, because di buku harian interrupts the verb and its object. The listener might momentarily wonder: write in the diary what? The phrase sounds awkward and unnatural.

So the common pattern is:

Subject – Verb – Object – Place
Saya menulis kenangan penting di buku harian.

Is Saya menulis kenangan penting the same as “I write about important memories”? Should there be a tentang?

There’s a subtle difference:

Saya menulis kenangan penting.
Literally: I write important memories.
It suggests you’re writing the memories themselves down (turning them into written form).
Saya menulis tentang kenangan penting.
Literally: I write about important memories.
This suggests you’re writing about them (maybe describing or discussing them), not just recording them.

In the context of a diary, you usually write the memories themselves, so:

Saya menulis kenangan penting di buku harian supaya tidak lupa.

feels very natural. Adding tentang would shift the nuance to “write about important memories” instead of “record the important memories.” Both are understandable; they just give slightly different mental images.

Could I say kemampuan mengingat or memori penting instead of kenangan penting?

You could, but they don’t mean the same thing:

kenangan penting = important memories (personal past experiences).
memori penting – uses the loanword memori; more technical or formal, often used in psychological or computer contexts (e.g., memory in the brain, computer memory). For personal, emotional memories, kenangan is more natural.
kemampuan mengingat = the ability to remember (a general ability, not specific memories).

Your sentence is about personal memories, so kenangan penting is the best and most natural choice.