Learn Hardcore Croatian: Treba mi drugi ključ za sobu.

Treba mi + NOMINATIVE literally means “(it) is needed to me,” with an impersonal verb and a dative experiencer: Treba mi drugi ključ (od/za sobu).
Trebam + ACCUSATIVE is the personal form “I need”: Trebam drugi ključ (od/za sobu).

In everyday Croatian, the impersonal Treba mi… is very common and sounds natural and neutral. Trebam… is also accepted and widely used. Choose freely; nuance is minor (Treba mi can sound a touch softer/less direct).

What case is mi, and what does it express?

Mi is the unstressed dative singular of the pronoun “I” (ja), meaning “to me/for me.” It marks the experiencer/beneficiary with this verb: Treba mi… = “(something) is needed to me.”

Dative full (stressed) form for emphasis: meni (e.g., Meni treba… “I, specifically, need…”).
Don’t confuse with accusative me/mene, used as a direct object with other verbs.

Why does mi come right after Treba? Can it move?

Mi is a clitic (an unstressed short form) and Croatian clitics prefer the “second position” in the clause. The default placement is after the first stressed element:

Treba mi drugi ključ za sobu.

You can move things around for focus/emphasis, but the clitic still wants second position:

Za sobu mi treba drugi ključ.
With emphasis on the person, use the full form first: Meni treba drugi ključ za sobu.

What case is drugi ključ here, and why not drugog ključa?

With the impersonal Treba mi, the “needed thing” is in the nominative: drugi ključ. You would not use genitive drugog ključa here.

Compare:

Impersonal: Treba mi novi prijatelj. (nominative; note it’s not “prijatelja”)
Personal: Trebam novog prijatelja. (accusative)

In our sentence, even with the personal form, masculine inanimate accusative looks like nominative: Trebam drugi ključ.

Does drugi mean “another” or “second”? When should I use još jedan?

Drugi can mean “second (in order)” or “the other (of two).” In many contexts it also works as “another (different)” key.
Još jedan literally means “one more,” emphasizing an additional item without implying it’s the other of two.

Nuance:

If you just want an extra key: Treba mi još jedan ključ (od/za sobu).
If you mean “a different key” or “the second key (of two)”: Treba mi drugi ključ (od/za sobu).

Why is it drugi (masculine) and not druga or drugo?

Agreement. Ključ is a masculine noun, so the modifier must be masculine: drugi ključ. You’d use druga with feminine nouns (e.g., druga soba) and drugo with neuter nouns (e.g., drugo pismo).

Why sobu and not soba? What case is used after za?

Za takes the accusative to express purpose/target. Soba (fem.) becomes sobu in the accusative singular:

za sobu = “for the room (intended for use in that room).”

Is za sobu the best preposition here, or should I say od sobe?

Both occur, with a nuance:

ključ od sobe (genitive) = “the key to the room,” i.e., the room’s key (ownership/connection). This is very idiomatic with “key.”
ključ za sobu (accusative) = “a key intended for the room” (purpose).

In practice, when talking about hotel room keys, ključ od sobe is very common: Treba mi drugi ključ od sobe.

Could I just say Treba mi drugi ključ and omit the room part?

Yes, if the context makes the target obvious (e.g., you’re standing at hotel reception). Without context, adding od/za sobu clarifies what the key is for.

How can I make this more polite when speaking to hotel staff?

Some polite options:

Molim vas, mogu li dobiti još jedan/drugi ključ od sobe?
Možete li mi, molim vas, dati još jedan/drugi ključ od sobe?
Softened need: Trebao/Trebalo bi mi još jedan/drugi ključ od sobe, molim vas.

Why is ključ masculine? How do its forms change?

Ključ is masculine. Useful forms:

Nominative sg.: ključ
Accusative sg. (inanimate): ključ (same as nominative)
Genitive sg.: ključa
Nominative pl.: ključevi
Accusative pl. (inanimate): ključeve
Genitive pl.: ključeva

Examples:

Treba mi ključ.
Trebam dva ključa / Trebaju mi dva ključa.

Should I say Treba mi dva ključa or Trebaju mi dva ključa?

Both are heard:

Impersonal pattern (very common): Treba mi dva ključa.
Personal agreement with the plural subject: Trebaju mi dva ključa.

In everyday speech, both are acceptable. The impersonal treba is extremely frequent.

Is there a difference between Treba mi and Nedostaje mi?

Yes:

Treba mi = “I need (something).” Focus on necessity.
Nedostaje mi = “I’m missing/lacking (something)” or “I miss (someone).” Focus on absence.

For a hotel key, both can work, but Treba mi (još jedan/drugi) ključ od sobe is the straightforward “I need” request.

How do I pronounce ključ and the special letters?

ključ ≈ “klyooch”: lj is a palatal “ly” sound [ʎ]; č is “ch” as in “church.”
treba: stress on the first syllable, “TRE-ba.”
drugi: “DROO-gee” (g as in “go”).

Are there natural word-order variations of the whole sentence?

Yes; you can shift elements for emphasis while keeping the clitic second:

Neutral: Treba mi drugi ključ od/za sobu.
Emphasis on “for the room”: Za sobu mi treba drugi ključ.
Emphasis on “me”: Meni treba drugi ključ od/za sobu.

All are fine; choose based on what you want to highlight.