Jika kamu kehilangan kunci, ambillah kunci cadangan di resepsionis.

The particle -lah softens or politely emphasizes a command.

• Ambil = neutral imperative (“take”).
• Ambillah = polite/softer, or slightly emphatic (“please take” / “do take”).

It’s common in written instructions and polite speech. In very casual contexts, people often just say ambil, or use ambil saja (“just take it”).

Mixing kamu (informal “you”) with ambillah (politer tone) is understandable but slightly mixed in register. More consistent options:

• Informal: Jika kamu kehilangan kunci, ambil kunci cadangan di resepsionis.
• Polite/formal: Jika Anda kehilangan kunci, ambillah kunci cadangan di resepsionis.
• Neutral sign-style (no pronoun): Jika kehilangan kunci, ambillah kunci cadangan di resepsionis.

Yes:

• Jika and bila are more formal.
• Kalau is more colloquial.

All mean “if” and can head the clause:
Jika/Kalau/Bila kamu kehilangan kunci, …
They’re interchangeable here; choose based on desired formality.

• Hilang = “to be gone/lost” (state). Example: Kuncinya hilang. (“The key is missing.”)
• Kehilangan (ke- -an) = “to lose (something),” typically unintentionally, focusing on the experiencer. Example: Saya kehilangan kunci. (“I lost my key.”)

Your sentence emphasizes the person losing the key, so kehilangan fits well.

All are possible, with nuance:

• di resepsionis = “at reception/with the receptionist” (location-focused; very common idiomatically).
• di meja resepsionis / di bagian resepsionis = explicitly “at the reception desk/section.”
• dari resepsionis = “from the receptionist” (source-focused).
• Avoid ke resepsionis with ambil; use it only with a motion verb: Pergi ke resepsionis untuk mengambil kunci.

In Indonesian, modifiers usually follow the noun.

• kunci cadangan = “spare key” (correct)
• cadangan kunci would mean “a reserve of keys” or sound odd in this context.

Yes:

• kunci serep (very common, informal–neutral)
• kunci duplikat (duplicate key; implies a copy)
• kunci cadangan (neutral–formal; good in signage)

Yes. Dropping kamu/Anda is natural in instructions and signs:

• Jika kehilangan kunci, ambillah kunci cadangan di resepsionis.
  The subject “you” is understood from context.

They’re all fine, with different shades:

• Ambillah kunci cadangan … = polite imperative.
• Silakan ambil kunci cadangan … = inviting/permission (“please go ahead and take…”).
• Tolong ambil kunci cadangan … = requesting help (“please help by taking…”).

For signage, Silakan ambil… or Ambil kunci cadangan… are very common.

Yes:

• Kalau kuncinya hilang, … = “If the key is lost, …” (focus on the key’s state).
• Kalau kamu kehilangan kunci, … = “If you lose the key, …” (focus on your experiencing the loss).

Both are acceptable; choose based on focus.

Indonesian often omits possessive markers when context makes possession clear. Kunci here is naturally understood as “your key.” You can add possession if needed:

• Informal: kuncimu
• Polite: kunci Anda Example: Jika Anda kehilangan kunci Anda, … is grammatical but usually the second Anda is dropped as redundant.

Use kalian for plural “you” (informal):

• Jika kalian kehilangan kunci, ambil kunci cadangan di resepsionis. In formal contexts, Anda can be singular or plural, so no change is needed:
• Jika Anda kehilangan kunci, ambillah kunci cadangan di resepsionis.