Kalau galon di dispenser sudah kosong, kakak perempuan saya memesan galon baru lewat aplikasi.

Kalau usually means if. In this sentence, it introduces the condition:

• Kalau galon di dispenser sudah kosong = If the water jug in the dispenser is already empty

In everyday Indonesian, kalau is very common in speech and writing. It can sometimes also mean when in a casual sense, depending on context, but here if/when is the natural idea.

Sudah often means already. Here it shows that the state of being empty has been reached:

• sudah kosong = already empty

Without sudah, kosong just means empty.
With sudah, the sentence emphasizes that the gallon has now become empty.

Compare:

• galon itu kosong = the gallon is empty
• galon itu sudah kosong = the gallon is already empty

In Indonesian daily use, galon often refers to the large refillable water container used for drinking water dispensers, not just the measurement gallon in English.

So here, galon means something like:

• water jug
• water bottle
• refill container for a dispenser

That is why the sentence later says memesan galon baru: it means ordering a replacement water container.

Di is a preposition meaning in, at, on, depending on context. Here:

• galon di dispenser = the gallon in/on the dispenser

This is di as a preposition, so it is written separately from dispenser.

That is different from the prefix di- used for passive verbs, which is written together, for example:

• dipesan = ordered

So:

• di dispenser = in/on the dispenser
• dipesan = is ordered / was ordered

Indonesian often specifies age and gender more explicitly.

• kakak = older sibling
• perempuan = female
• kakak perempuan saya = my older sister

This is different from English, where sister does not tell you whether she is older or younger.

Related words:

• adik perempuan = younger sister
• kakak laki-laki = older brother
• adik laki-laki = younger brother

So kakak perempuan saya is very natural if the speaker wants to say my older sister specifically.

The base/root is pesan, which relates to ordering or booking.
The form memesan is an active verb built with the meN- prefix:

• pesan = order / message / to order, depending on context
• memesan = to order

In a full standard sentence, memesan sounds more grammatical and complete:

• Kakak perempuan saya memesan galon baru = My older sister orders a new gallon

In casual speech, Indonesians may sometimes say just pesan, but memesan is the more standard form here.

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

So memesan by itself can mean:

• orders
• is ordering
• ordered
• will order

The time is understood from context.

In this sentence, because of the kalau clause, it sounds like a habitual or general action:

• If the gallon in the dispenser is empty, my older sister orders a new one through the app.

But in the right context, it could also be understood as a past event. Indonesian relies heavily on context rather than verb conjugation.

Baru usually means new. In this sentence:

• galon baru = a new gallon

But here new does not necessarily mean brand-new in the factory sense. It more likely means another/replacement gallon to replace the empty one.

So in natural English, depending on context, this could be understood as:

• a new gallon
• another gallon
• a replacement water jug

Lewat here means through or via:

• lewat aplikasi = through/via an app

Yes, it is very similar to melalui aplikasi.

The difference is mostly style:

• lewat aplikasi = more everyday, conversational
• melalui aplikasi = a bit more formal

Both are natural. In casual modern Indonesian, lewat aplikasi is very common.

Indonesian does not normally use articles like English a/an/the.

So:

• galon can mean a gallon, the gallon, or just gallon
• dispenser can mean a dispenser or the dispenser

The listener figures it out from context.

In this sentence:

• galon di dispenser is naturally understood as the gallon in the dispenser
• galon baru is understood as a new gallon

So Indonesian leaves this unstated unless extra clarity is needed.

Yes. Indonesian is flexible, especially with condition clauses.

Original:

• Kalau galon di dispenser sudah kosong, kakak perempuan saya memesan galon baru lewat aplikasi.

You could also say:

• Kakak perempuan saya memesan galon baru lewat aplikasi kalau galon di dispenser sudah kosong.

Both are understandable. The original version puts the condition first, which is very natural when setting up the situation before the main action.

Kosong is usually treated like an adjective meaning empty.

So:

• galon itu kosong = the gallon is empty

Indonesian does not always require a separate verb like is in English. That is why sudah kosong works directly without a word meaning is.

Literally, the structure is closer to:

• the gallon already empty

But in natural English, we translate it as:

• the gallon is already empty

Yes, sometimes kalau can feel like when in practical meaning, especially in habitual situations.

For example, this sentence could imply a repeated pattern:

• When the gallon in the dispenser is empty, my older sister orders a new one through the app.

So grammatically kalau is still basically a conditional word, but in real use it can overlap with when if the speaker is describing what usually happens.