Learn Hardcore Polish: Nauczyciel mówi, że żeby mieć dobrą ocenę, trzeba powtarzać materiał częściej. - The teacher says that to have a good grade, one must review the material more often.

Nauczyciel mówi = The teacher says / is saying (now, or in general).
It often introduces something that is still valid as a rule, opinion, or instruction.
Nauczyciel powiedział = The teacher said (at some specific time in the past).

In this sentence, the teacher is giving a general guideline about how to get a good grade, so Polish naturally uses the present tense mówi to present it as a current, valid piece of advice, not just something that happened once in the past.

Why do we have both że and żeby together: mówi, że żeby mieć…? Isn’t one of them enough?

They introduce two different subordinate clauses:

że = that (after verbs like mówi, myśli, wie)
It starts the content of what the teacher says.
żeby = roughly (in order) to / so that
It introduces a clause of purpose or goal.

Structure:

Nauczyciel mówi, że [żeby mieć dobrą ocenę, trzeba powtarzać materiał częściej].
= The teacher says that [to get a good grade, one must revise the material more often].

You could also hear:

Nauczyciel mówi, żeby mieć dobrą ocenę, trzeba powtarzać materiał częściej.

Here że is dropped; this is more like:

The teacher tells us (that) to get a good grade, you must…

So:

że links to mówi (he says that…)
żeby introduces the purpose (in order to have a good grade…)

Both together are very natural when reporting what someone says about what needs to be done to achieve a goal.

Who is the subject of mieć and powtarzać? There is no I/you/we in the sentence.

There is no explicit subject; it is generic (like English one / you / people).

The structure is:

żeby mieć dobrą ocenę = in order to have a good grade
trzeba powtarzać materiał częściej = one must / you have to revise the material more often

In Polish, with trzeba + infinitive, we usually understand the subject as:

one / you / we / people in general

Similarly, in English you might say:

To get a good grade, you have to revise more often.

Even if you don’t literally mean you personally, but students in general. Polish leaves this implied, and trzeba plus the infinitive already carries the meaning of a general obligation.

What exactly does żeby do in żeby mieć dobrą ocenę?

Here żeby introduces a purpose clause with an infinitive:

żeby mieć dobrą ocenę = (in order) to have a good grade

With żeby + infinitive:

It often means in order to.
The (logical) subject of the infinitive is understood from context.
Here it is the same generic one/you as with trzeba.

Compare:

Żeby zdać egzamin, trzeba się uczyć.
To pass the exam, you have to study.

So żeby is not “that” here; it is “so as to / in order to” with a goal-oriented meaning.

Why is it dobrą ocenę and not dobra ocena?

Because dobrą ocenę is in the accusative singular (feminine), as the direct object of mieć (to have).

Nominative (dictionary form):
- dobra ocena = a good grade (subject)
  e.g. Dobra ocena cieszy ucznia. – A good grade makes the student happy.
Accusative (object of the verb):
- dobrą ocenę = a good grade (object)
  e.g. Chcę mieć dobrą ocenę. – I want to have a good grade.

Pattern (feminine noun ocena + adjective dobry):

Nominative: dobra ocena
Accusative: dobrą ocenę

In żeby mieć dobrą ocenę, mieć takes a direct object in the accusative, so both the adjective and noun change to dobrą ocenę.

What does trzeba mean exactly, and how is it different from musieć?

Trzeba is an impersonal expression of necessity:

trzeba + infinitive ≈ one must / it is necessary to / you have to

In this sentence:

trzeba powtarzać materiał częściej
≈ you/one have to revise the material more often

Key points:

trzeba has no personal forms (I trzeba, ty trzeba do not exist).
It is always just trzeba.
The “doer” is understood from context (here: students in general).

Musieć, on the other hand, is a normal verb meaning must / have to:

Muszę powtarzać materiał częściej. – I must revise the material more often.
Musisz powtarzać materiał częściej. – You must revise the material more often.

So:

trzeba = impersonal, generic obligation
musieć = personal, conjugated, talks about a specific person or group

Why is powtarzać used here, and not powtórzyć?

Powtarzać and powtórzyć are an imperfective / perfective pair:

powtarzać (imperfective) – to repeat / to revise (ongoing, repeated, habitual action)
powtórzyć (perfective) – to repeat / to revise once, to complete the repetition

In the sentence:

trzeba powtarzać materiał częściej

This describes a regular habit (“revise more often”), not a single one-time action. That’s why the imperfective powtarzać is used.

If you wanted a one-off future action, you could say, for example:

Żeby lepiej zrozumieć temat, trzeba będzie powtórzyć materiał.
In order to understand the topic better, it will be necessary to go over the material (once).

So here powtarzać fits because the teacher is talking about a general study habit.

What does materiał mean in this context? Is it like English “material(s)”?

Yes, materiał in school/university context is very close to English study material / course content / subject matter.

powtarzać materiał = revise the material / go over the material again

Some notes:

materiał is singular here, used in a collective sense, like English material (uncountable).
You can also use materiały (plural) when you mean physical teaching materials (handouts, files, slides):
materiały z lekcji = handouts / materials from class.

Here, materiał simply means all the content that has been taught and that you’re supposed to revise.

What form is częściej, and how does it relate to często?

Częściej is the comparative form of the adverb często:

często = often
częściej = more often

Pattern:

base adverb: często
comparative: częściej (ending -ej is typical for many comparative adverbs)

So:

powtarzać materiał często = to revise the material often
powtarzać materiał częściej = to revise the material more often (than now / than before / than others)

In the sentence, częściej expresses that the current frequency is not enough; it should be higher.

Why are there so many commas, and could the word order be different?

Polish uses commas more extensively than English, especially:

Before subordinating conjunctions like że, żeby:
- Nauczyciel mówi, że…
- …że żeby mieć dobrą ocenę, trzeba…
Around the introductory purpose clause:
- żeby mieć dobrą ocenę, trzeba powtarzać materiał częściej

Possible variations in word order (all grammatical, with slightly different emphasis):

Żeby mieć dobrą ocenę, trzeba częściej powtarzać materiał.
Żeby mieć dobrą ocenę, materiał trzeba powtarzać częściej.
Trzeba powtarzać materiał częściej, żeby mieć dobrą ocenę.

The basic rules that stay:

Comma before że and before żeby when they start subordinate clauses.
The clause żeby mieć dobrą ocenę is separated by commas from the main clause with trzeba.

The chosen version is a very natural, neutral word order for written Polish.