Learn Hardcore Croatian: Na početku sata ukratko ponavljamo sadržaj prošlog predavanja. - At the beginning of the lesson we briefly revise the content of the previous lecture.

The basic form is sat (nominative singular).
In the phrase na početku sata, sata is genitive singular, governed by the noun početak.

Think of it this way:

početak čega? – the beginning of what? → početak sata (the beginning of the class).
Then we put početak into the locative (na početku), but sata stays in the genitive because it depends on početak, not on na.

So the internal structure is:

na početku (locative of početak)
sata (genitive, dependent on početak – beginning of the class)

What’s the difference between sat and predavanje in this sentence?

Both relate to teaching, but they’re not the same:

sat – a class/lesson period (often 45 minutes in school).
- Here: na početku sata = at the beginning of the class (period).
predavanje – a lecture (usually at university or in a more formal setting).
- Here: sadržaj prošlog predavanja = the content of the last lecture.

So the idea is:
At the beginning of this class, we briefly review the content of the previous lecture.

Why is it ukratko and not kratko? Are they different?

Both are adverbs from kratak (short), but they differ in usage and nuance:

kratko = shortly / briefly in a more literal sense (for a short time).
- E.g. Kratko smo razgovarali. – We talked for a short time.
ukratko = briefly, in brief, in short – more about summarizing.
- E.g. Ukratko, nema problema. – In short, there’s no problem.
- In your sentence: ukratko ponavljamo = we briefly/briefly (in summary) review.

In this context, ukratko sounds more natural because you’re summarizing the earlier lecture, not just doing something for a short amount of time. Kratko ponavljamo is not wrong, but ukratko is idiomatic here.

What verb form is ponavljamo, and what is the infinitive?

Ponavljamo is:

1st person plural, present tense, imperfective aspect
from the infinitive ponavljati – to repeat, to review, to revise.

Aspectually:

ponavljati (imperfective) – ongoing/repeated action: to be reviewing / to review (regularly).
Perfective partner: ponoviti – a single, completed repeat: to repeat once / to go over once.

In your sentence, ponavljamo suggests a regular routine: At the beginning of the class we (normally) review…

Could we use ponovimo instead of ponavljamo? What would change?

Yes, grammatically you can say:

Na početku sata ukratko ponovimo sadržaj prošlog predavanja.

Ponovimo is:

1st person plural, present (often used as a polite suggestion or instruction),
from ponoviti (perfective).

Difference in nuance:

ponavljamo – describes a habitual action: what we generally do at the start of each class.
ponovimo – sounds like a proposal or instruction in this particular lesson: Let’s briefly go over the content of the last lecture now.

So the original fits a description of a regular classroom routine.

Why do we say sadržaj prošlog predavanja and not something like prošli sadržaj predavanja?

In sadržaj prošlog predavanja:

sadržaj – content (subject, nominative)
prošlog predavanja – of the last lecture (genitive phrase modifying sadržaj)

The question is: sadržaj čega? → content of what? → prošlog predavanja.

We want to say the content of the last lecture, not the previous content of the lecture. English also prefers content of the last lecture, not last content of the lecture.

If you say prošli sadržaj predavanja, it sounds like:

prošli sadržaj = the previous content (content itself is previous),
which is odd or misleading. The lecture is last/previous, not its content.

Why is it prošlog predavanja and not prošle predavanje?

Two things are happening:

Case:
- We need the genitive because of sadržaj (čega?) – content of what?
- So predavanje (lecture) goes to genitive singular:
  - nominative: predavanje
  - genitive: predavanja
Adjective agreement:
- prošli (last) must agree with predavanje in case, number, and gender.
- predavanje is neuter singular.
- Neuter singular genitive for prošli is prošlog.

So:

nominative: prošlo predavanje – the last lecture
genitive: prošlog predavanja – of the last lecture

Prošle predavanje is wrong because:

prošle is not the correct form for neuter singular genitive.

Why is predavanja in the genitive case here?

Because it depends on sadržaj (content). The logic is:

sadržaj – content
čega? (of what?) – this triggers the genitive.

Answer to čega?:

prošlog predavanja – of the last lecture

So predavanja is genitive singular, governed by the meaning content of X. This is very common:

naslov knjige – the title of the book (genitive)
početak filma – the beginning of the film (genitive)
sadržaj prošlog predavanja – the content of the last lecture

Can I say Na početku časa instead of Na početku sata?

In standard Croatian:

sat is the normal word for a class/lesson period.
čas is much more typical in Serbian and Bosnian.

So:

In Croatia, Na početku sata is the natural standard form.
Na početku časa will be understood, but it sounds more Serbian/Bosnian than Croatian.

If you’re specifically learning Croatian, stick with sat for school/class context.

Can the word order change, for example Ukratko na početku sata ponavljamo…?

Croatian word order is relatively flexible, but some options are more natural.

Original:

Na početku sata ukratko ponavljamo sadržaj prošlog predavanja.

Other natural variants:

Na početku sata ponavljamo ukratko sadržaj prošlog predavanja.
Ukratko ponavljamo sadržaj prošlog predavanja na početku sata.

These are all understandable; the main difference is which part you emphasize (time, brevity, or action).

However:

Ukratko na početku sata ponavljamo… is possible but sounds slightly marked/stylistic; it makes ukratko stand out more at the beginning.

The original order (time phrase first, then adverb, then verb) is very typical and neutral.

Is Na početku sata closer to At the beginning of the hour or At the beginning of the class?

In this context, it’s clearly At the beginning of the class/lesson.

Croatian sat can mean:

hour (60 minutes, time unit), or
lesson/class period in a school or course timetable.

Here, because you’re reviewing the content of a previous lecture, it refers to the teaching period: At the beginning of the class…, not just at the beginning of the hour in a purely clock sense.