If
Kalau hujan, saya tidak pergi. ‘If it rains, I won’t go.’
Kalau mau, boleh. ‘If you want, sure.’
No verb changes, no subjunctive, no tense agreement. The condition is marked by kalau and nothing else moves.
And also when
Indonesian does not separate if from when in the way English does.
Kalau sudah sampai, telepon saya. ‘When you arrive, call me.’
Whether it is a condition or a certainty is left to the situation. Ketika and waktu exist for ‘when’ specifically, and they belong to writing and to the past.
As for
The use nobody teaches and everybody uses: kalau introduces a change of topic, roughly like the English “as for”.
Kalau saya, saya lebih suka teh. ‘As for me, I prefer tea.’
Kalau harganya? ‘And the price?’
This is a soft way to redirect a conversation, and it is worth having early because it lets you ask about something without appearing to interrogate.
Otherwise
Kalau tidak, kita naik taksi. ‘Otherwise, we’ll take a taxi.’
In speech this becomes kalau nggak, and often just nggak gitu.
The threat version
Nanti placed in the second half turns a condition into a warning, in a way English does with “or you’ll”.
Kalau tidak makan, nanti sakit. ‘If you don’t eat, you’ll get ill.’
This construction is everywhere in speech between family members, and it sounds far less severe in Indonesian than the English translation suggests.
The formal versions
Jika and apabila both mean if and belong to contracts, signs and formal writing. Recognise them, do not say them.
What the book says: Jika Anda memerlukan bantuan, silakan hubungi kami.
What you’ll hear: Kalau butuh apa-apa, bilang aja.