Saya menulis di buku harian, dan saya merasa lebih tenang setelahnya.

Indonesian often repeats the subject after dan if the speaker wants the sentence to sound clear and complete.

So:

• Saya menulis di buku harian, dan saya merasa lebih tenang setelahnya.

is very natural.

You could also say:

• Saya menulis di buku harian dan merasa lebih tenang setelahnya.

That is also understandable, because the subject of both verbs is still saya. Repeating saya just makes the structure a little clearer.

Literally, it means to write in a diary/journal.

Breaking it down:

• saya = I
• menulis = write / am writing
• di = in / on / at
• buku harian = diary, journal

So menulis di buku harian is literally write in a diary.

Indonesian does not always need a direct object in the same way English does. In English, we often say write in a diary, and Indonesian works similarly here.

Because di is the normal preposition for location: in, on, or at.

Here, the idea is writing in the diary, so di buku harian is natural.

• di buku harian = in the diary

The other options do not fit as well here:

• pada buku harian is not the usual choice for this meaning
• ke buku harian means movement to the diary, which is not what is meant

So for a learner, the simplest rule is: if you mean writing in a diary, use di buku harian.

Buku harian literally means daily book:

• buku = book
• harian = daily

Together, it means diary or journal.

It is a very common expression. Depending on context, English might translate it as:

• diary
• journal

Indonesian often leaves out possessive words when the meaning is already obvious from context.

So:

• Saya menulis di buku harian
  can naturally mean
• I write in my diary

Even without my, listeners usually understand that it is the speaker’s diary.

If you want to make it explicit, you can say:

• buku harian saya = my diary

So both are possible:

• Saya menulis di buku harian.
• Saya menulis di buku harian saya.

The version without saya after buku harian is a little lighter and more natural when ownership is obvious.

Merasa means to feel.

In this sentence:

• saya merasa lebih tenang = I feel calmer / I feel more calm

It is commonly followed by an adjective:

• merasa senang = feel happy
• merasa sedih = feel sad
• merasa lelah = feel tired
• merasa tenang = feel calm

So merasa works a lot like English feel before an adjective.

In Indonesian, lebih means more.

So:

• tenang = calm
• lebih tenang = more calm

In natural English, we usually say calmer, but Indonesian does not change the adjective itself. It just adds lebih.

Compare:

• besar = big
• lebih besar = bigger / more big
• cepat = fast
• lebih cepat = faster
• tenang = calm
• lebih tenang = calmer

So lebih + adjective is a very important pattern for comparisons.

Setelahnya means after that, afterward, or after it.

It comes from:

• setelah = after
• -nya = a suffix that can refer back to something already mentioned

In this sentence, -nya refers to the act of writing in the diary.

So:

• setelahnya = after that / afterward

A very natural English translation here is afterward.

Here, -nya points back to something already known from the context.

So:

• setelah = after
• setelahnya = after it / after that / afterward

In this sentence, it refers to the earlier action: writing in the diary.

This is common in Indonesian. -nya can sometimes mean:

• his/her/their
• the
• it/that, depending on context

Here it is closest to that or it.

Yes. Sesudahnya is also possible and means almost the same thing: afterward or after that.

• setelahnya = afterward
• sesudahnya = afterward

In many situations, they are interchangeable. Setelah is often a little more common in everyday use, but both are correct.

Indonesian verbs do not change form for tense the way English verbs do.

So:

• menulis can mean write, am writing, wrote, depending on context
• merasa can mean feel, am feeling, felt, depending on context

This sentence is often understood as a general or present statement, especially in isolation:

• I write in my diary, and I feel calmer afterward.

But in the right context, it could also describe a past habit or repeated action.

Indonesian usually relies on context or time words if tense needs to be clearer.

The comma helps separate two related clauses:

• Saya menulis di buku harian
• dan saya merasa lebih tenang setelahnya

In informal Indonesian, people may or may not use the comma here. It is not unusual to write the sentence without it:

• Saya menulis di buku harian dan saya merasa lebih tenang setelahnya.

So the comma is fine, but it is not something you need to worry too much about as a beginner.

Yes. The original sentence is already natural, but there are a few alternatives depending on style.

For example:

• Saya menulis di buku harian, dan saya merasa lebih tenang setelahnya.
• Saya menulis di buku harian dan merasa lebih tenang setelahnya.
• Saya menulis di buku harian saya, dan setelah itu saya merasa lebih tenang.

All of these are good. The differences are small:

• repeating saya sounds clearer
• adding saya after buku harian makes my diary explicit
• setelah itu is a slightly more transparent way to say after that

So the original sentence is perfectly fine, and these are just useful variations to know.