U poruci je objašnjeno da će domaća zadaća biti kratka, ali da mora biti uredna.

Because u meaning in takes the locative case when it expresses location.

• Base form: poruka = message
• Locative singular: poruci
• So u poruci = in the message

This is a very common pattern in Croatian:

• u školi = in school
• u knjizi = in the book
• u poruci = in the message

If u meant movement into something, it would usually take the accusative instead.

Because objašnjeno does not describe poruka.

Here, u poruci is just telling you where something was explained. The core structure is:

• je objašnjeno = it was explained

This is an impersonal passive-like construction. Croatian often uses the neuter singular form in this kind of sentence when the real content comes afterward in a da-clause.

So the logic is roughly:

• U poruci je objašnjeno...
• In the message, it was explained...

The thing being explained is the whole clause that follows:

• da će domaća zadaća biti kratka...

So objašnjeno is neuter singular because it works like English it was explained, not because it agrees with poruka.

Here da introduces a content clause, similar to English that.

So:

• je objašnjeno da... = it was explained that...

In this sentence, da introduces what was explained:

• da će domaća zadaća biti kratka
• that the homework will/would be short

This use of da is extremely common in Croatian after verbs of saying, knowing, explaining, thinking, etc.

For example:

• Znam da dolazi. = I know that he/she is coming.
• Rekla je da nema vremena. = She said that she doesn’t have time.
• Objasnili su da je važno. = They explained that it is important.

Because there are two coordinated clauses, and both depend on je objašnjeno.

So the structure is:

• je objašnjeno da će domaća zadaća biti kratka
• ali da mora biti uredna

Repeating da makes the structure clearer and more balanced. It shows that both ideas are part of what was explained.

In English, we often do the same:

• It was explained that the homework would be short, but that it had to be neat.

Could Croatian sometimes omit the second da? In some contexts, yes, but here repeating it is very natural and standard.

That is the future tense.

Croatian future I is normally formed with:

• a form of htjeti as an auxiliary: ću, ćeš, će, ćemo, ćete, će
• plus the infinitive

Here:

• će biti = will be

So:

• domaća zadaća će biti kratka = the homework will be short

After da, the auxiliary usually comes right after it:

• da će domaća zadaća biti kratka

That placement is normal because će is a clitic-like auxiliary and tends to appear near the beginning of the clause.

Because mora biti expresses a requirement more directly: it must be neat.

• mora biti = must be / has to be
• će morati biti = will have to be

In this sentence, the speaker is presenting a rule or requirement about the homework, not mainly focusing on the future moment of obligation. So mora biti uredna sounds more natural.

Compare:

• Zadaća će biti kratka. = The homework will be short.
• Zadaća mora biti uredna. = The homework must be neat.

You could say da će morati biti uredna, but that would shift the nuance more toward it will have to be neat, which is a bit heavier and less simple.

Because they all relate to zadaća, which is a feminine singular noun.

Croatian adjectives must agree with the noun in:

• gender
• number
• case

So:

• zadaća = feminine singular
• domaća zadaća = feminine singular adjective + feminine singular noun
• kratka = feminine singular
• uredna = feminine singular

That is why you see the -a ending on all of them.

If the noun were masculine, the forms would change:

• kratak
• uredan

If it were neuter:

• kratko
• uredno

Because it is the subject of the clause:

• da će domaća zadaća biti kratka

In that subordinate clause, the homework is the thing that will be short, so it stays in the nominative.

The conjunction da does not automatically force a different case. Case depends on the noun’s role in the clause.

Here:

• domaća zadaća = subject
• therefore nominative

Domaća zadaća is a standard Croatian expression for homework.

Literally:

• domaća = home, domestic
• zadaća = task / assignment

So the phrase literally means something like home assignment, but in normal usage it simply means homework.

You may also hear just:

• zadaća

when the context already makes it clear that it is homework.

Yes. In this context, uredna means something like:

• neat
• tidy
• well-presented

When talking about schoolwork, uredna zadaća usually means the homework should be clean, organized, and carefully written.

So it is not about being short only; it also has to be presented properly.

Yes, Croatian word order is flexible, but not completely free.

The sentence as given is natural and neutral:

• U poruci je objašnjeno da će domaća zadaća biti kratka, ali da mora biti uredna.

You could rearrange some parts for emphasis, but you must be careful with clitics like je and će, which have preferred positions.

For example, je in je objašnjeno and će in da će... are not placed randomly.

So while Croatian allows more flexibility than English, the given order is a very good standard model for learners.

Yes. Croatian often allows both an impersonal/passive-like style and a more active style.

For example:

• U poruci su objasnili da će domaća zadaća biti kratka, ali da mora biti uredna.

That means roughly:

• In the message, they explained that the homework would be short, but that it had to be neat.

The original version with je objašnjeno is more impersonal and often sounds more formal or neutral. It avoids naming who did the explaining.

So:

• je objašnjeno = impersonal, neutral
• su objasnili = active, with an implied they