Learn Hardcore Croatian: Paket je već poslan, ali još nije stigao. - The package has already been sent, but it still hasn’t arrived.

Paket je već poslan = je helps form a passive/result state (has been sent / is already sent).
(Paket) još nije stigao = je is again the auxiliary for the past/perfect-like form (hasn’t arrived yet). In the second clause it appears inside nije (ne + je), meaning is not / has not.

Is poslan a past tense form? Why not use a normal verb like poslao?

poslan is a past passive participle (masculine singular), used to express that the package is in the state of having been sent.

Paket je poslan = The package has been sent / was sent (focus on the package as the receiver of the action). If you used an active form like poslao, you would need a sender:
Netko ga je poslao = Someone sent it.

Why is it poslan (masculine) and not poslana or poslano?

The participle agrees in gender and number with the subject paket:

What exactly do već and još mean, and why are both used?

They create a contrast in time:

već = already (the sending happened earlier than expected or is completed)
još = still / yet (the arrival has not happened up to now) So već … ali još nije … often maps to already … but still hasn’t … / but hasn’t … yet.

Why is još used with a negative: još nije stigao?

With negatives, još commonly means still:

još nije stigao = he/it still hasn’t arrived Croatian can also use još in the sense of yet with a negative, especially in translations, but the most natural feel is still (not).

How does negation work in nije stigao?

Negation attaches to the auxiliary:

je = is/has
nije = is not / has not Then the participle stays affirmative:
stigao (arrived) doesn’t change form; the negativity is carried by nije.

What is stigao grammatically? Is it the same kind of form as poslan?

Both are participle-like forms, but they come from different systems:

stigao is an active past participle (masculine singular) from stići (to arrive). It forms the perfect-like past with biti: je stigao / nije stigao.
poslan is a passive participle from poslati (to send) used in a passive/result construction: je poslan (has been sent). So they look similar but express different voice (active vs passive).

Does this sentence mean was sent or has been sent?

Croatian Paket je poslan can cover both English readings depending on context:

has been sent (relevant now; status update)
was sent (simple past narrative) The adverb već often pushes the “status update” feeling: already sent.

Can the word order change? For example: Paket je poslan već or Još paket nije stigao?

Yes, Croatian word order is flexible, but some orders sound more natural:

Paket je već poslan is the most neutral.
Paket je poslan već is possible but usually sounds more emphatic/marked (often used in speech to stress already).
Paket još nije stigao is very common (placing the subject again in the second clause).
Još paket nije stigao is possible but tends to sound more stylistic/emphatic.

Why is there a comma before ali?

Because two independent clauses are being joined with the coordinating conjunction ali (but):

Paket je već poslan, ali još nije stigao. This is standard Croatian punctuation.

Can you omit the second paket? How do we know what nije stigao refers to?

Yes, it’s normal to omit it because the subject is understood from the first clause:

Paket je već poslan, ali još nije stigao. Croatian often drops repeated subjects when they are obvious from context.

How would I say it if I want to sound more “formal” or more “spoken”?

Common variants:

Neutral/formal: Paket je već poslan, ali još nije stigao.
More explicit: Paket je već poslan, ali paket još nije stigao.
Very spoken: Paket je već poslan, ali još nije došao. (došao = came, often used informally for things/people)

What’s the difference between stigao and došao here?

stići → stigao focuses on arriving/reaching the destination (often used for deliveries, deadlines, transport).
doći → došao is come, more general and conversational. For a package, stigao sounds slightly more precise/logistics-like, while došao sounds more casual.