Ono što je mladoj ženi najvažnije jest da na vjenčanju svi budu opušteni i da nitko ne kasni.

Ono što is a very common Croatian structure meaning something like what or the thing that.

So:

• Ono = that / that thing
• što = which / what / that

Together, ono što literally means that which, but in natural English it is usually just what.

In this sentence:

• Ono što je mladoj ženi najvažnije...
• literally: That which is most important to the young woman...
• natural English: What is most important to the young woman...

Croatian often uses this two-part structure where English would simply say what.

Because mladoj ženi is in the dative case, not the nominative.

The adjective and noun are:

• nominative: mlada žena = a young woman
• dative: mladoj ženi = to/for a young woman

In this sentence, the idea is:

• something is most important to her

Croatian often expresses important to someone with the dative:

• To mi je važno. = That is important to me.
• Njemu je to jasno. = That is clear to him.
• Mladoj ženi je to najvažnije. = That is most important to the young woman.

So mladoj ženi is dative because it marks the person for whom something is important.

Because it agrees with ono što, which functions as a neuter singular idea.

Here the subject is not a masculine or feminine noun like čovjek or žena, but the whole abstract idea ono što = what / the thing that. In Croatian, that is treated as neuter singular, so the predicate adjective is also neuter singular:

• važno = important
• važnije = more important
• najvažnije = most important

So:

• Ono što je... najvažnije...

not:

• najvažnija (feminine)
• najvažniji (masculine)

because the grammatical subject is effectively neuter.

Jest is the full form of je (is).

Both are correct:

• je = the usual short unstressed form
• jest = the full, stressed form

In this sentence:

• Ono što je mladoj ženi najvažnije jest da...

Using jest sounds a bit more formal, careful, or emphatic. It is especially common in written language when introducing an explanation or definition.

You could also say:

• Ono što je mladoj ženi najvažnije je da...

That would be completely normal too.

Because there are two separate subordinate clauses, both depending on jest.

The structure is:

• ... jest da na vjenčanju svi budu opušteni
• i da nitko ne kasni

So the sentence means:

• ... is that everyone be relaxed at the wedding
• and that nobody be late

Croatian often repeats da before each coordinated clause. This is very natural and usually clearer than leaving it out.

After da, Croatian often uses the present tense form in a way that corresponds to English ideas like:

• that they be
• that they are
• for them to be

So:

• da svi budu opušteni

literally looks like that everyone are relaxed, but functionally it means:

• that everyone be relaxed
• for everyone to be relaxed

Using su here would not fit this construction.

Compare:

• Svi su opušteni. = Everyone is relaxed.
  (plain statement)

• Važno je da svi budu opušteni. = It is important that everyone be relaxed / for everyone to be relaxed.
  (subordinate clause after da)

So budu is the correct form in this da-clause.

In Croatian, svi can mean:

• all
• everyone
• everybody

depending on context.

Here, there is no noun after it, so it means everyone/everybody:

• svi budu opušteni = everyone is / be relaxed

Also notice agreement:

• svi is grammatically plural
• so the verb is plural: budu
• and the adjective is plural: opušteni

That is why you get:

• svi budu opušteni

not singular forms.

In Croatian, the masculine plural form is normally used for mixed groups or when gender is not specified.

So:

• opušteni can mean relaxed for a group of men, or a mixed group, or a general everyone
• opuštene would be specifically feminine plural

Since svi refers to everyone at the wedding, Croatian uses the default plural form:

• svi budu opušteni

This is standard grammar, not a statement that the group is male.

Vjenčanju is in the locative singular.

The preposition na can take different cases depending on meaning:

• na + accusative = movement onto/to something
• na + locative = location or event where something happens

Here it means at the wedding, so Croatian uses na + locative:

• na vjenčanju = at the wedding

Compare:

• Idem na vjenčanje. = I’m going to the wedding.
  (na + accusative, movement)

• Na vjenčanju je bilo lijepo. = It was nice at the wedding.
  (na + locative, location/event)

Because Croatian uses negative concord: if the subject is a negative word like nitko (nobody), the verb is also negative.

So standard Croatian is:

• nitko ne kasni = nobody is late / nobody arrives late

not:

• nitko kasni

This is very different from standard English, where double negatives are usually avoided. In Croatian, this is normal and required:

• Nitko ne zna. = Nobody knows.
• Ništa ne vidim. = I see nothing.
• Nikada ne dolazi. = He never comes.

So nitko ne kasni is exactly what you should expect.

Kasni is the present tense of the imperfective verb kasniti = to be late / to arrive late / to run late.

In this sentence, after da, the present tense expresses a desired or important situation:

• da nitko ne kasni = that nobody be late / that nobody arrives late

It does not necessarily mean present time only. In this kind of clause, it often expresses a general intended result or expectation.

Grammatically, mladoj ženi simply means to the young woman.

It does not literally mean bride. The usual Croatian words for bride are:

• mladenka
• nevjesta

However, in context — especially since the sentence mentions a wedding — the person could of course be the bride. But that would come from the situation, not from the phrase mladoj ženi itself.

So the grammar says young woman; the wider context might suggest the bride.

Yes. Croatian word order is fairly flexible, although some versions sound more natural than others.

The given sentence is very natural and well balanced:

• Ono što je mladoj ženi najvažnije jest da na vjenčanju svi budu opušteni i da nitko ne kasni.

You could also say:

• Mladoj ženi je najvažnije da na vjenčanju svi budu opušteni i da nitko ne kasni.

This is slightly simpler and probably more common in everyday speech.

The version with ono što puts special focus on what is most important and sounds a bit more formal or stylistically polished.

The sentence has three main parts:

1. Ono što je mladoj ženi najvažnije
   = What is most important to the young woman

2. jest
   = is

3. da na vjenčanju svi budu opušteni i da nitko ne kasni
   = that everyone be relaxed at the wedding and that nobody be late

So the whole structure is:

• What is most important to X is that Y ...

This is a very useful pattern in Croatian:

• Ono što mi je najvažnije jest da...
• Ono što im je potrebno jest da...
• Ono što želi jest da...

It is a common formal way to introduce the most important point.