Learn Hardcore Malay: Pada waktu malam, pantai menjadi gelap kerana matahari sudah terbenam. - At night, the beach becomes dark because the sun has already set.

Pada waktu malam literally means “at nighttime.”
Malam by itself can mean “night” or “evening.”
Waktu malam emphasizes the “time of night” (“the nighttime period”).
Native speakers often shorten pada waktu malam to malam or di malam hari in casual speech.

Why is pada used before waktu malam? Could we use di or omit the preposition?

Pada is a general preposition for time expressions (e.g., pada hari Selasa, pada pukul 8).
Di is normally used for places (di pantai, di sekolah), though colloquially you might hear di malam hari.
You can omit the preposition in informal contexts and just say waktu malam or simply malam, but pada is more formal/complete.

What is the function of menjadi in pantai menjadi gelap? Could we say pantai gelap instead?

Menjadi means “to become.” It shows a change of state.
Pantai menjadi gelap = “the beach becomes dark.”
Pantai gelap would be “the beach is dark,” which is fine if you only want to describe the state without implying the change.

Why use menjadi instead of the shorter jadi?

Menjadi is slightly more formal and is a true intransitive verb (“to become”).
Jadi can also mean “become,” but it’s more colloquial and has other senses (“so,” “then,” or “okay”).
If you say pantai jadi gelap, it’s understood, but menjadi is clearer in writing.

What does kerana mean, and are there alternatives?

Kerana means “because.” It’s the more formal/written connector.
Informal alternatives: sebab (conjunction) or kerana itu (“for that reason”).
Example: “Pantai gelap sebab matahari terbenam.”

In matahari sudah terbenam, what role does sudah play?

Sudah means “already.”
It marks that the action (the sun setting) is complete by the mentioned time.
Without sudah, matahari terbenam is just “the sun sets” (habitual or general).

Could we replace sudah with telah? Any difference?