Learn Hardcore Croatian: Za njen novi posao pripremamo malo iznenađenje i obiteljsko slavlje. - For her new job we are preparing a small surprise and a family celebration.

njen novi posao is in the accusative after za.
(For masculine inanimate nouns like posao, nominative and accusative have the same form, so it looks like nominative but is syntactically accusative.)

Examples:

za mene (for me – accusative)
za sutrašnji sastanak (for tomorrow’s meeting – accusative)

Is njen the same as njezin? Which one is more correct?

Both njen and njezin mean “her” and are grammatically correct.

njezin is often considered a bit more formal/standard in grammar books.
njen is very common in everyday speech and also accepted.

You could say:

Za njen novi posao…
Za njezin novi posao…

Both are fine; style guides may favor njezin, but native speakers use njen all the time.

Why is it njen novi posao and not something like njena novi posao?

The possessive pronoun njen agrees with posao, not with the owner (the woman).

posao = masculine singular noun
The adjective/pronoun before it must be masculine singular too:
- njen (her – masc. sg.)
- novi (new – masc. sg.)

So:

njen novi posao = her new job (masc. sg. + masc. sg. + masc. sg.)

If the noun were feminine, it would change:

njena nova kuća (her new house – kuća is feminine)

Is njen novi posao nominative or accusative in this sentence?

Grammatically it is accusative, because it follows za, and za always takes accusative.

You can’t see a form change here because:

posao (masc. inanimate) has the same form in nominative and accusative singular:
- Nominative: posao je težak (the job is hard)
- Accusative: imam novi posao (I have a new job)

In Za njen novi posao…, it is functioning as accusative, even though it looks like nominative.

What exactly does pripremamo express here? Is it like “we prepare” or “we are preparing”?

Pripremamo is:

1st person plural, present tense of pripremati (imperfective aspect)
It can correspond to “we prepare”, “we are preparing”, or “we’re going to prepare”, depending on context.

In this sentence:

Za njen novi posao pripremamo malo iznenađenje…
≈ We are preparing / We’re planning a little surprise…

Croatian doesn’t have a separate continuous tense like English, so present covers both we prepare and we are preparing. Future intent is often understood from context.

Could you also say something like pripremit ćemo here? What would be the difference?

Yes:

Pripremamo… – present tense, often used for ongoing or planned action, can sound a bit more immediate or “in progress”.
Pripremit ćemo… – future tense (perfective pripremiti
- ćemo), more explicitly future-oriented:
  - We will prepare a little surprise…

Both are natural.

Pripremamo – sounds like you’re already in the process or it’s a current plan.
Pripremit ćemo – emphasizes the future action as a single event.

What is malo doing in malo iznenađenje? Is it an adjective or an adverb here?

In malo iznenađenje, malo functions as an adjective meaning “small, little”:

iznenađenje = surprise (neuter singular)
malo agrees with it in gender and number (neuter singular: -o ending).

So malo iznenađenje = a small/little surprise.

Compare with other uses of malo:

Adjective (agreeing with a noun):
- malo dijete (a small child – neuter)
- mala kuća (a small house – feminine)
- mali grad (a small town – masculine)
Adverb/quantifier meaning “a little”:
- Malo sam umoran. (I’m a little tired.)
- Popit ću malo vode. (I’ll drink a little water.)

In your sentence it’s clearly the adjectival “small”.

Why is it obiteljsko slavlje, not obiteljska slavlje?

Because the adjective must agree with slavlje in gender, number, and case.

slavlje = celebration, a neuter singular noun in Croatian.
The adjective obiteljski (family) must take the neuter singular form: obiteljsko.

So:

obiteljsko slavlje (family celebration – neuter)
Not:
obiteljska slavlje (wrong, obiteljska is feminine singular)

Compare:

obiteljska večera (family dinner – večera is feminine)
obiteljski auto (family car – auto is masculine)
obiteljsko slavlje (family celebration – slavlje is neuter)

What are iznenađenje and slavlje in terms of gender, and how does that affect the adjectives?

Both are neuter singular nouns:

iznenađenje (surprise) – neuter sg.
slavlje (celebration) – neuter sg.

The adjectives before them must therefore be neuter singular:

malo iznenađenje – malo: neuter sg.
obiteljsko slavlje – obiteljsko: neuter sg.

Typical neuter singular adjective endings in the nominative/accusative are -o or -e:

novo iznenađenje, dobro slavlje, veliko dijete.

Is malo iznenađenje i obiteljsko slavlje one object or two? How does the coordination work?

They are two separate direct objects of the same verb pripremamo:

pripremamo [malo iznenađenje] i [obiteljsko slavlje].

Both are in the same case (accusative, as objects), and joined by i (and). Croatian doesn’t need any extra endings or agreement on the verb for coordinated objects; the structure is straightforward.

Can I change the word order, for example: Pripremamo malo iznenađenje i obiteljsko slavlje za njen novi posao? Is that still correct?

Yes, that word order is grammatically correct and very natural:

Pripremamo malo iznenađenje i obiteljsko slavlje za njen novi posao.

Both versions are fine:

Za njen novi posao pripremamo malo iznenađenje i obiteljsko slavlje.
Pripremamo malo iznenađenje i obiteljsko slavlje za njen novi posao.

The difference is mostly one of emphasis:

Starting with Za njen novi posao puts more focus on the occasion/reason: As for her new job…
Starting with Pripremamo puts more focus on what you’re doing: We are preparing…

Croatian word order is fairly flexible; main constraints involve placing clitics, which don’t appear in this sentence.

Does this sentence sound formal, informal, or neutral? Would anything change in a very formal context?

It sounds neutral, suitable for everyday conversation, a message, or even light written text.

For a more formal style, you might:

Prefer njezin over njen:
- Za njezin novi posao pripremamo malo iznenađenje i obiteljsko slavlje.

Otherwise, the sentence is already quite neutral and appropriate in most contexts.