Learn Hardcore Indonesian: Di buku harian, saya menulis bahwa hati manusia bisa rapuh tetapi juga kuat ketika bersama teman. - In my diary, I wrote that the human heart can be fragile but also strong when together with friends.

di is used for a location, including abstract ones like a page, a book, a website:
- di buku harian – in the diary
- di koran – in the newspaper
- di internet – on the internet

You could also sometimes find “dalam buku harian” (inside the diary), but “di buku harian” is more natural and common in this context.

What does “buku harian” mean, and is it the normal way to say “diary”?

“Buku harian” literally means “daily book”, but idiomatically it means “diary / journal”.

buku = book
harian = daily (from hari = day)

In everyday Indonesian, “buku harian” is a very common and natural way to say “a diary”. There is also “diari” (from English/French “diary”), but “buku harian” sounds more neutral and widely used.

What is the function of “saya menulis bahwa …”, and could you omit “bahwa”?

“Saya menulis bahwa …” = “I wrote that …”

saya = I (formal/neutral)
menulis = wrote / to write
bahwa = “that” (introduces a clause, like English that in I wrote that…)

So the structure is:

Saya menulis bahwa + [clause]
= I wrote that + [clause]

Can you omit “bahwa”?

Yes, often you can:

Saya menulis hati manusia bisa rapuh …

is usually still understandable, but:

Using bahwa makes the sentence clearer and more formal.
In writing (especially more formal or careful style), bahwa is very common.

Spoken, informal Indonesian might drop bahwa more often.

Does “saya menulis” mean past tense “I wrote” or present tense “I write”? There is no tense marking here.

Indonesian verbs do not change form for tense. “menulis” by itself can mean:

I write
I am writing
I wrote
I was writing

The tense is understood from context or from time words:

Kemarin saya menulis di buku harian.
Yesterday I wrote in my diary.
Setiap hari saya menulis di buku harian.
Every day I write in my diary.

In your sentence:

Di buku harian, saya menulis bahwa …

In English we translate it naturally as “I wrote that …”, because diaries are usually written about past experiences or reflections. But grammatically, Indonesian doesn’t force a specific tense.

Why is “hati manusia” used here, and does “hati” mean “heart” or “liver”?

“Hati manusia” means “the human heart” (in an emotional sense).

hati can mean both “liver” and “heart”, depending on context.
When talking about feelings, emotions, character, hati is understood as “heart”.

Examples:

sakit hati – heartbroken, deeply hurt (emotionally)
dari lubuk hati yang paling dalam – from the deepest part of my heart
hati yang lembut – a gentle heart

So “hati manusia bisa rapuh tetapi juga kuat” =
“the human heart can be fragile but also strong”, clearly about emotions, not anatomy.

What does “bisa” mean here, and how is it different from “dapat” or “mungkin”?

In this sentence, “bisa” means “can / is able to / is capable of”.

bisa rapuh = can be fragile
bisa kuat = can be strong

Differences:

bisa = can / able to (most common, everyday)
dapat = can / may / able to (a bit more formal or written; also used in rules, permissions)
mungkin = maybe / perhaps (expresses possibility, not ability)

You could say:

hati manusia dapat rapuh tetapi juga kuat – grammatically fine, a bit more formal.
mungkin rapuh would change the meaning to “may be fragile / perhaps is fragile”, not “can be”.

What does “rapuh” mean exactly, and is it about emotions or physical things?

“Rapuh” means “fragile / easily broken / brittle / delicate”.

It can be used for:

Physical things:
- kaca ini rapuh – this glass is fragile
- tulangnya rapuh – his/her bones are brittle
Emotional/mental state:
- mental yang rapuh – fragile mental state
- hati yang rapuh – a fragile heart (emotionally vulnerable)

In your sentence:

hati manusia bisa rapuh
= the human heart can be fragile (emotionally)

What is the nuance of “tetapi” here, and how is it different from “tapi”?

Both “tetapi” and “tapi” mean “but / however”.

tetapi – more formal / written / careful
tapi – more informal / spoken / casual

Your sentence:

… bisa rapuh tetapi juga kuat …

is stylistically a bit more formal or neutral.

In casual speech, many people would say:

… bisa rapuh tapi juga kuat …

Meaning is the same; it’s mostly a difference of style/level of formality.

Why is “juga” placed before “kuat”? Could it be placed elsewhere?

“Juga” means “also / too / as well”.

Here:

… bisa rapuh tetapi juga kuat …
= … can be fragile but also strong …

Position:

Common and natural: tetapi juga kuat
You can move juga slightly, but the most idiomatic placement in this pattern is before the adjective it modifies:
- bisa rapuh tetapi juga bisa kuat – adds a repeated bisa, slightly more emphatic.
- bisa rapuh, tetapi kuat juga – possible, more colloquial/emphatic, like “but strong as well”.

The original placement “tetapi juga kuat” is smooth, natural, and clearly links “strong” back to “fragile” as a contrast.

What does “ketika” mean here, and how is it different from “saat” or “waktu”?

“Ketika” means “when” (introducing a time clause).

… kuat ketika bersama teman.
= … strong when (it is) together with friends.

Comparisons:

ketika – when (neutral), used in many written and spoken contexts.
saat – when / at the time (a bit more formal/literary, but also common in speech).
waktu – literally “time”, but also used as “when” in more informal speech.

You could say:

kuat ketika bersama teman
kuat saat bersama teman
kuat waktu bersama teman (more colloquial)

All three are understandable, with slight differences in tone; ketika is a good neutral choice here.

How should I understand “bersama teman”? Why isn’t there “my” (like “my friends”)?

“Bersama teman” literally = “together with friends”.

bersama = together with
teman = friend / friends (Indonesian doesn’t always mark plural)

There is no explicit “my” (saya / aku / ku-) in the phrase. Indonesian often omits possessive pronouns when the context is clear.

In context, “bersama teman” will usually be understood as “with (one’s) friends”, or “when (a person) is with friends”.

If you want to be explicit:

bersama teman-teman saya – with my friends (plural, explicit)
bersama teman saya – with my friend(s) (singular in English, but can still be plural in context)

Why is the comma placed after “Di buku harian”? Is that normal in Indonesian?

Yes, that comma is normal and good style.

The phrase “Di buku harian” is an introductory prepositional phrase indicating where the writing happens. In Indonesian writing, it’s common (and clearer) to separate such intro phrases with a comma:

Di sekolah, saya belajar bahasa Indonesia.
Pada pagi hari, kami berolahraga.

So:

Di buku harian, saya menulis bahwa …

The comma functions like in English:

In my diary, I wrote that …

Could “saya” be replaced with “aku” here, and would that change the tone?

Yes, you could say:

Di buku harian, aku menulis bahwa …

Both are grammatically correct, but:

saya – more formal / neutral / polite, safe in almost any context
aku – more informal / intimate, used with friends, family, or in personal writing

Since this is about a diary, using “aku” might feel more personal and intimate, which is common in actual diary entries.

However, the original sentence might be from a more neutral narration, so “saya” is also completely natural.