Learn Hardcore Malay: Jika sambungan internet hilang, saya tekan butang kecil untuk muat turun semula. - If the internet connection is lost, I press a small button to download again.

Jika means “if” and is the formal conditional connector.
Kalau is its informal equivalent, so you can say Kalau sambungan internet hilang… in casual speech.
Apabila means “when” and implies the event is expected to happen. It’s less suitable for a pure “if” condition, though you might hear Apabila sambungan internet hilang… if you’re treating the loss as inevitable.

What exactly does sambungan internet hilang mean, and why use hilang instead of terhilang?

Sambungan internet hilang literally means “the internet connection disappears” or “is lost.”
Hilang is the active verb “to disappear/be lost.”
Terhilang suggests an unintentional or accidental loss (e.g. losing an item). Since a dropped connection is more about something stopping actively, hilang is preferred. You could also say sambungan internet terputus (“the connection is cut/disconnected”).

Why does the speaker say saya tekan rather than saya menekan?

In colloquial Malay, verbs often appear in their root form without the meN- prefix, especially in speech and informal writing.
Saya tekan is perfectly natural in everyday conversation.
Saya menekan is the fully affixed, formal version. Both mean “I press.”

Why is butang kecil placed immediately after tekan, and why isn’t there a word for “the” or “a”?

Malay follows a Subject-Verb-Object order, so tekan (press) is followed by its object butang kecil (small button).
Malay has no articles like “a” or “the.” The noun phrase butang kecil can mean “a small button,” “the small button,” or just “small buttons,” depending on context.

What is the function of untuk in untuk muat turun semula? Could I use another word?

Untuk expresses purpose: “in order to.”
Here it links the pressing action to its goal: press the button in order to download again.
You could replace it with supaya (“so that”), e.g. saya tekan butang kecil supaya muat turun semula, but untuk is more concise.

What does muat turun semula mean, and why is semula at the end?

Muat turun is the Malay verb for “download” (literally “load down”).
Semula means “again.”
In Malay, words like semula indicating repetition typically follow the main verb or verb phrase: muat turun semula (“download again”).

How would I turn that into passive voice: “If the internet connection is lost, the small button is pressed to download again”?

In passive Malay you use the di- prefix on the verb.
A passive version is:
Jika sambungan internet hilang, butang kecil ditekan untuk muat turun semula.
Here ditekan means “is pressed.” Passive constructions are less common in everyday Malay but are grammatically correct.