Na ploči je napisano da danas ponavljamo nominativ i akuzativ.

Because na here means on in the sense of location, so it requires the locative case.

• ploča = board
• na ploči = on the board

This is a very common pattern in Croatian:

• na stolu = on the table
• na zidu = on the wall
• na ploči = on the board

If na expresses movement onto something, it usually takes the accusative instead:

• Stavljam knjigu na stol. = I am putting the book onto the table.

But here there is no movement, only location, so locative is used.

Je napisano literally means is written or has been written.

It comes from:

• je = is
• napisano = written

In this sentence, it has an impersonal/passive-like meaning:

• Na ploči je napisano... = It is written on the board...

Croatian often uses this structure when English uses something like it says or it is written.

So the sentence is not focusing on who wrote it. The important thing is only that the message is there on the board.

Because this is an impersonal construction.

In English, we often say:

• It is written...
• It says...

Croatian does something similar by using the neuter singular past participle:

• napisano

This neuter form is very common in impersonal expressions, especially when there is no specific noun that the participle agrees with.

So:

• Na ploči je napisano... = It is written on the board...

You do not need to imagine a hidden noun like poruka or rečenica here. The expression works on its own.

Here da means that.

It introduces a subordinate clause:

• Na ploči je napisano = It is written on the board
• da danas ponavljamo nominativ i akuzativ = that today we are reviewing the nominative and accusative

So the full structure is:

• It is written on the board that today we are reviewing the nominative and accusative.

This is a very common use of da in Croatian.

Ponavljamo means we review / we are reviewing / we repeat.

It is the 1st person plural form of ponavljati / ponoviti, depending on context, but here the meaning is clearly to review / go over again in class.

The we usually refers to the teacher and students together, or just the class in a general sense:

• danas ponavljamo = today we are reviewing

Croatian often uses we in classroom contexts where English would also naturally say we.

Because Croatian verb endings already show the subject.

• ponavljam = I review
• ponavljaš = you review
• ponavljamo = we review

So ponavljamo already contains the idea of we.

You can add mi for emphasis:

• Mi danas ponavljamo nominativ i akuzativ.

But in a neutral sentence, it is usually omitted.

Because they are being used here as the names of grammar cases, not as ordinary nouns in some other grammatical role.

After ponavljati in the sense of reviewing/studying, Croatian often uses the direct object:

• ponavljamo nominativ i akuzativ

So these words function as objects of the verb, but since they are also the standard names of cases, they appear in their usual dictionary forms here.

In practice, learners should understand this as:

• We are reviewing the nominative and accusative.

This is normal classroom language.

It is present tense, but Croatian present tense can correspond to different English present meanings depending on context.

So ponavljamo can mean:

• we review
• we are reviewing
• we’re going over again

In this sentence, because of danas and the classroom context, the most natural translation is usually:

• Today we are reviewing the nominative and accusative.

Yes, Croatian word order is fairly flexible, and danas can often be moved.

The given sentence says:

• Na ploči je napisano da danas ponavljamo nominativ i akuzativ.

This is natural and neutral.

You could also hear variations such as:

• Na ploči je napisano da ponavljamo danas nominativ i akuzativ.
• Danas na ploči je napisano da ponavljamo nominativ i akuzativ.

But those may sound less natural depending on emphasis.

The original version is good because danas naturally modifies ponavljamo and comes early in the subordinate clause.

Yes, absolutely.

A very natural English translation is:

• It says on the board that today we are reviewing the nominative and accusative.

Even though napisano literally means written, English often prefers it says in this kind of situation.

So possible translations include:

• It is written on the board that...
• It says on the board that...
• Written on the board is the message that...
  
  • This last one is possible, but much less natural.

Yes.

• nominativ i akuzativ = the nominative and the accusative

There is nothing unusual about i in this sentence. It is the regular Croatian coordinating conjunction and.