Cijela posada čeka skipera na palubi.

Because cijela has to agree with posada in gender, number, and case.

• posada = a feminine singular noun
• so the adjective must also be feminine singular nominative
• that gives cijela

Compare:

• cijela posada = the whole crew
• cijeli brod = the whole ship
  (because brod is masculine)

So this is standard adjective–noun agreement.

Because posada is a collective noun. In Croatian, words like posada are grammatically singular even if they describe a group of people.

So:

• posada čeka = the crew is waiting

not

• posada čekaju

This is similar to English the crew is waiting, where crew is also grammatically singular in many contexts.

For the same reason: the subject is posada, which is grammatically singular.

• čeka = he/she/it waits or is waiting
• here it agrees with posada

So the structure is:

• Cijela posada = subject
• čeka = 3rd person singular present
• skipera = object
• na palubi = location

If the subject were plural, you would use čekaju:

• Mornari čekaju skipera. = The sailors are waiting for the skipper.

Because skiper is the direct object of čeka, so it must be in the accusative case.

skiper is a masculine animate noun, and masculine animate nouns usually change in the accusative singular:

• nominative: skiper
• accusative: skipera

This is very common in Croatian:

• vidim čovjeka = I see the man
• čekam prijatelja = I’m waiting for a friend
• čeka skipera = is waiting for the skipper

If the noun were masculine inanimate, nominative and accusative would usually be the same.

Because this sentence describes location, not movement.

With na, Croatian often uses:

• locative for being at/on a place
• accusative for movement toward a place

So:

• na palubi = on the deck / on deck
  (location, so locative)
• na palubu = onto the deck
  (movement, so accusative)

In your sentence, the crew is already there, waiting. That is why paluba becomes palubi.

It is the locative singular of paluba.

The base noun is:

• paluba = deck

After the preposition na when it means on in the sense of location, Croatian uses the locative:

• na palubi = on the deck

So:

• nominative: paluba
• locative: palubi

It can mean both, depending on context.

Croatian present tense often covers both:

• waits
• is waiting

So Cijela posada čeka skipera na palubi can mean:

• The whole crew waits for the skipper on deck
• The whole crew is waiting for the skipper on deck

English chooses between simple present and present continuous, but Croatian often just uses the present tense form and lets context do the work.

Because Croatian does not have articles like English a/an/the.

So nouns appear without an article:

• posada can mean a crew or the crew
• skiper can mean a skipper or the skipper
• paluba can mean a deck or the deck

Definiteness is understood from context.

In this sentence, English naturally uses the whole crew, the skipper, and the deck, but Croatian does not need separate words for the.

Yes. Croatian word order is fairly flexible because case endings show what each word is doing.

The neutral order here is:

• Cijela posada čeka skipera na palubi.

But other orders are possible, for example:

• Na palubi cijela posada čeka skipera.
  This emphasizes where they are.
• Skipera cijela posada čeka na palubi.
  This emphasizes who they are waiting for.

Even when the order changes, the endings still show the grammar:

• posada = subject
• skipera = object
• palubi = location after na

So word order often affects focus and emphasis, not basic meaning.

Skiper is a normal and commonly used word, especially in nautical contexts. It is a borrowing, but it is fully integrated into Croatian.

It is not always exactly the same as kapetan:

• skiper often means the person steering or leading a smaller boat or yacht
• kapetan is more like captain, often more formal or official

In many everyday boating contexts, skiper sounds very natural.

Yes. čekati is an imperfective verb.

That means it presents the action as:

• ongoing
• repeated
• in progress
• without emphasizing completion

So in this sentence, čeka simply describes the ongoing situation: the crew is waiting.

Croatian often contrasts imperfective and perfective verbs. With waiting, different perfective verbs can add slightly different meanings, for example:

• pričekati = wait a bit / wait for a while
• dočekati = wait until someone arrives; receive someone on arrival

But in a plain descriptive sentence like this one, čekati is the natural choice.