Na satu bilježimo nove riječi u bilježnicu.

Na satu is a very common Croatian expression meaning in class / during the lesson.

• sat can mean hour, but in a school context it also means class period / lesson
• after na in the sense of location or setting, Croatian often uses the locative
• satu is the locative singular of sat

So:

• na satu = in class / during the lesson

Why not u satu?

• u satu would sound more like in the hour or within an hour, which is a different idea
• Croatian idiomatically says na satu for school lessons

Croatian often leaves out subject pronouns when they are clear from the verb ending.

Here, bilježimo ends in -mo, which tells you the subject is we.

So:

• bilježimo = we write down / we are writing down

You could say Mi bilježimo..., but that usually adds emphasis, like:

• Mi bilježimo, a oni ne. = We are writing it down, but they aren’t.

In a neutral sentence, leaving out mi is more natural.

Bilježimo is the 1st person plural present tense form of bilježiti.

In this sentence, it means something like:

• we write down
• we note down
• we record

Because Croatian present tense often covers both simple and continuous meanings, bilježimo can correspond to:

• we write down
• we are writing down

depending on context.

Nove riječi is the direct object of the verb bilježimo, so it is in the accusative.

Breakdown:

• riječ = word
• plural = riječi = words
• nov = new
• feminine plural form = nove

Because riječ is a feminine noun, the adjective has to agree with it.

So:

• nove riječi = new words

A useful detail: for many feminine nouns, the nominative plural and accusative plural look the same, so riječi stays riječi here.

This is a classic Croatian case difference:

• u + accusative = movement into
• u + locative = position in

Here, the words are being written into the notebook, so Croatian uses the accusative:

• u bilježnicu = into the notebook

If you were talking about something already being inside the notebook, you would use the locative:

• u bilježnici = in the notebook

So compare:

• Pišemo riječi u bilježnicu. = We are writing words into the notebook.
• Riječi su u bilježnici. = The words are in the notebook.

Because Croatian prepositions change case depending on meaning.

In this sentence:

• na satu uses na + locative for the setting: in class
• u bilježnicu uses u + accusative for direction: into the notebook

So the sentence combines:

• where/when the action happens → na satu
• where the words are being put → u bilježnicu

This is very normal in Croatian. The case is not chosen by the preposition alone, but by the preposition + meaning together.

Yes. Croatian word order is more flexible than English because the case endings show what each word is doing.

The given sentence is perfectly natural:

• Na satu bilježimo nove riječi u bilježnicu.

But other orders are possible, for example:

• Bilježimo nove riječi u bilježnicu na satu.
• Nove riječi bilježimo u bilježnicu na satu.

The meaning stays basically the same, but the emphasis changes.

The original version starts with Na satu, which gives the setting first: In class, we write down new words in the notebook.

Good question. Bilježiti already has the idea of noting down / recording, so the sentence would still be grammatical without u bilježnicu:

• Na satu bilježimo nove riječi.

That would mean:

• In class, we write down new words.

Adding u bilježnicu simply makes it more specific:

• where do we write them down?
• into the notebook

So it is not strictly necessary, but it adds useful detail.

Yes, both are true.

Sat most commonly means hour, but in school contexts it also means a class period / lesson.

That is why:

• na satu can mean during the lesson / in class

This is similar to how English sometimes uses period or class depending on context.

So in school language:

• sat hrvatskog = Croatian class
• sat matematike = math class

The dictionary form is bilježnica, meaning notebook.

Here are the relevant forms:

• bilježnica = nominative singular
• bilježnicu = accusative singular
• bilježnici = locative singular

In this sentence you see bilježnicu because it follows u with the meaning of movement into something.

So:

• dictionary form: bilježnica
• form used in the sentence: bilježnicu