Learn Hardcore Indonesian: Saya sedih kalau dia tidak hadir.

kalau can mean both “if” (conditional) and “when/whenever” (time-related).
In this sentence it most naturally reads as a condition: you feel sad in the event that the person doesn’t attend. It can also be understood as a general rule (“whenever he doesn’t attend”).
Purely time-based “when” (for a specific past time) is better with ketika or waktu: Saya sedih ketika dia tidak hadir (kemarin).
More formal “if” words: jika, apabila. Colloquial: kalau.

Can I switch the clause order?

Yes:

Saya sedih kalau dia tidak hadir. (no comma needed)
Kalau dia tidak hadir, saya sedih. (use a comma after the initial clause) Meaning and tone are the same; starting with the kalau-clause slightly highlights the condition.

Why is it tidak and not bukan?

Use tidak to negate verbs, adjectives, and prepositional phrases. hadir is a verb (“be present/attend”), so tidak hadir is correct.
Use bukan to negate a noun or a noun-like identification: Dia bukan guru. (He is not a teacher.)

What’s the difference between hadir, datang, ada, and menghadiri?

hadir: to be present at an event/meeting; a bit formal or administrative. Dia tidak hadir.
datang: to come/arrive (movement). Dia datang terlambat.
ada: to be there/to exist (presence in a place, not specifically an event). Dia tidak ada di kantor.
menghadiri: to attend something (transitive; takes an object). Dia tidak menghadiri rapat. So your sentence focuses specifically on attendance/presence at an event.

Should I use karena instead of kalau if I mean an actual cause?

Use kalau for a condition (“if/whenever”).
Use karena for an actual cause (“because”): Saya sedih karena dia tidak hadir. That states a real reason, not a conditional possibility.

How do I show future or past time, since Indonesian doesn’t mark tense?

Add time words:

Future: Saya akan sedih kalau dia tidak hadir besok.
Past specific time: Saya sedih ketika/waktu dia tidak hadir tadi/kemarin.
Completed action marker: sudah is possible in some contexts, but with hadir you usually rely on time words.

Is kalau interchangeable with ketika or saat?

Not always.

kalau = if/when (conditional or habitual time).
ketika/saat = when (time only, not conditional). For a one-off past event, prefer ketika/saat. For a condition or habit, kalau is best.

What about formality: saya vs aku, dia vs ia/beliau, and kalau vs jika?

saya (neutral/formal), aku (informal/intimate).
dia (he/she, neutral), ia (more formal, mainly as subject), beliau (honorific “he/she” for respected people).
kalau (colloquial/neutral), jika/apabila (more formal), bila (common in Malay; understood in Indonesian).

How can I intensify or soften sedih?

Intensify: sangat sedih, sedih sekali, benar-benar sedih, colloquial sedih banget.
Soften: agak sedih, lumayan sedih, cukup sedih.
Different nuance: kecewa (disappointed), terluka (hurt), murung (gloomy).

How do I say “whenever he doesn’t attend, I’m sad”?

Use a habitual marker:

Saya sedih setiap kali dia tidak hadir. Your original with kalau can also imply a general rule, but setiap kali makes the habitual meaning explicit.

How do I express a more hypothetical or contrary-to-fact idea?

Use seandainya/andai/andaikan for hypothetical conditions, and often add akan:

Seandainya dia tidak hadir, saya akan sedih. For “even if,” use kalaupun:
Kalaupun dia tidak hadir, saya tetap tenang. (Concessive, not the same meaning as your sentence.)

Can I drop pronouns in casual speech?

Often yes, if context is clear:

Sedih kalau dia tidak hadir. (Sounds like casual “(I’m) sad if he doesn’t attend.”) In careful writing, keep Saya. Dropping dia is less common unless already obvious from context.

Is dia gendered?

No. dia can mean “he” or “she.” Context tells you which. If you need clarity, use a name or title: kalau Budi tidak hadir / kalau ibu itu tidak hadir.

Are there more colloquial or alternative negatives?

Colloquial: nggak/enggak/gak. Example: … kalau dia nggak hadir.
Literary/concise (more Malay-ish but also used in Indonesian writing): tak. Example: … jika dia tak hadir.

Does kalau tidak ever mean “otherwise”? Is that what’s happening here?

Kalau tidak alone can mean “otherwise”: Cepat selesaikan, kalau tidak saya marah.
In your sentence, kalau directly links to dia tidak hadir, so it’s a standard conditional “if he doesn’t attend,” not the standalone “otherwise.”

Can I use a noun form like “absence” here?

Yes, with a more formal tone:

Saya sedih atas ketidakhadirannya. (“I’m sad about his/her absence.”)
ketidakhadiran = absence; -nya = his/her. You can also say Saya sedih karena ketidakhadirannya.

How does hadir compare with everyday choices like datang?

Everyday speech often uses datang (“come”): Saya sedih kalau dia tidak datang. This is very natural.
hadir emphasizes being present/attending an event (common in school/office contexts, roll calls, invitations). Both are acceptable; pick based on tone and context.

Any quick pronunciation tips?

Saya: SAH-yah
sedih: suh-DEEH
kalau: kah-lau (lau as in “allow” without the initial a)
dia: DEE-ah
tidak: TEE-dak (final k is a glottal stop)
hadir: hah-DEER (rolled or tapped r)