Razrednica kaže da u školskom dnevniku piše tko danas nije došao na nastavu.

Razrednica is the feminine form of razrednik / razrednica, which means a homeroom teacher, form teacher, or class teacher.

• razrednik = male homeroom teacher
• razrednica = female homeroom teacher

So the sentence tells you that the teacher in question is a woman.

Here da means that and introduces a subordinate clause.

• Razrednica kaže... = The homeroom teacher says...
• da u školskom dnevniku piše... = that it says in the school register...

In English, that is often optional, but in Croatian da is very commonly used in this structure.

Because u can take different cases depending on meaning.

Here it means in a place, so it takes the locative:

• školski dnevnik = school register/logbook
• u školskom dnevniku = in the school register/logbook

Both the adjective and the noun change:

• školski → školskom
• dnevnik → dnevniku

If u meant movement into something, you would use the accusative instead.

Not in the personal sense. Although dnevnik can mean diary or journal, in a school context školski dnevnik means the class register, attendance book, or school record book.

So this is not a student’s private diary. It is an official school record.

Here piše does not mean that someone is actively writing at this moment. It is being used in the sense of:

• it says
• it is written

So u školskom dnevniku piše... means something like:

• it says in the school register...
• it is written in the school register...

This is a very common Croatian way to refer to written information.

In standard Croatian, tko is the usual word for who.

• tko = standard Croatian
• ko = common in some colloquial speech and more usual in some other varieties of the language

So tko danas nije došao na nastavu is the standard form.

Nije došao is the negative perfect tense of doći and means did not come or has not come.

This is used because the sentence refers to a specific completed event: whether someone came to class today or not.

• doći = to come, to arrive once
• nije došao = did not come / has not come

By contrast:

• ne dolazi = is not coming / does not come
• nije dolazio = was not coming / did not use to come / kept not coming, depending on context

So nije došao is the natural choice for school attendance on a particular day.

Because tko is often treated grammatically as masculine singular when the person is unknown or unspecified, especially in the past tense.

So Croatian normally says:

• Tko je došao? = Who came?
• Tko nije došao? = Who didn’t come?

The masculine form works as the default here, even if the actual person could be female. This is a grammar pattern, not necessarily a comment about the person’s sex.

Because na here expresses movement toward an activity, so it takes the accusative:

• nastava = teaching, classes, lessons
• na nastavu = to class / to lessons

So doći na nastavu means to come to class or to attend lessons.

Compare:

• On je na nastavi. = He is in class.
  Here na nastavi is locative, because it describes location/state.

• On dolazi na nastavu. = He is coming to class.
  Here na nastavu is accusative, because it involves movement.

Not exactly.

• sat = one class period, one lesson, one hour of class
• nastava = instruction/classes/teaching as an activity or school attendance more generally

So nije došao na nastavu sounds like did not come to class / did not attend lessons, not necessarily just one single period.

That is normal Croatian word order. The sentence is built like this:

• Razrednica kaže
• da u školskom dnevniku piše
• tko danas nije došao na nastavu

Croatian word order is flexible, but this order sounds natural because it first introduces the place where the information is written, and then gives the content of that information.

A very literal breakdown is:

• The homeroom teacher says
• that in the school register it says
• who did not come to class today

Yes, depending on context, the Croatian perfect can sometimes overlap with English didn’t come and hasn’t come.

In a school-attendance context, though, the most natural interpretation is simply:

• didn’t come to class today
• was absent from class today

The surrounding situation usually makes the time meaning clear.