Learn Hardcore Indonesian: Tolong matikan lampu sebelum tidur. - Please turn off the light before sleeping.

Tolong is a polite marker equivalent to please in English. In an imperative request, you normally place Tolong at the beginning:
Tolong matikan lampu sebelum tidur.
You can also put it at the end for a slightly more casual tone:
Matikan lampu sebelum tidur, tolong.

Why is the verb matikan and not mematikan or mati?

The root verb is mati (to die / to go off).

mati alone is intransitive (“to die” or “the light goes off”).
mematikan is the active infinitive (“to turn off”).
In imperatives you drop the me- prefix and use the causative suffix -kan, so mati + -kan becomes matikan (“turn off (something)”).

What does the suffix -kan do in matikan?

The suffix -kan makes a verb transitive and causative. It changes mati (“to go off”) into matikan, meaning “cause to go off” or “turn off (something).”

Why is there no subject in Tolong matikan lampu sebelum tidur?

Indonesian imperatives typically omit the subject pronoun (kamu/Anda) because it’s understood you’re addressing “you.” Adding (Anda) or (kamu) is unnecessary:
• (Anda) tolong matikan lampu sebelum tidur.

What does sebelum mean in sebelum tidur, and can I move that phrase?

Sebelum means before. Sebelum tidur = before sleeping. As a time clause it’s flexible:
• At the end: Tolong matikan lampu sebelum tidur.
• At the start: Sebelum tidur, tolong matikan lampu.

Can I add -nya to lampu, as in matikan lampunya sebelum tidur?

Yes. Adding -nya often marks it as “that lamp” or makes it slightly more colloquial:
• Tolong matikan lampu sebelum tidur.
• Tolong matikan lampunya sebelum tidur.
Both are correct.

Is Tolong matikan lampu sebelum tidur polite enough for a formal sign?

It’s polite in everyday contexts. For a more formal notice (e.g., in a hotel) you might use Mohon or Harap instead of Tolong:
• Mohon matikan lampu sebelum tidur.
• Harap matikan lampu sebelum tidur.