Learn Hardcore Polish: Moja siostra szybko stresuje się ocenami, ale uczy się coraz lepiej. - My sister quickly gets stressed about grades, but she is learning better and better.

stresuje is the 3rd person singular present form: on/ona stresuje = he/she stresses (himself/herself)
się is a reflexive particle that is part of the verb.

Literally stresuje się is “(she) stresses herself,” but in natural English we just say “she gets stressed / she is easily stressed.”

You must include się with this verb when talking about a person being stressed; stresować without się usually means “to stress (someone/something)” in a more transitive sense, e.g. stresować uczniów – to stress the students out.

Why is it uczy się and not just uczy?

The base verb is uczyć się – to learn.

uczy on its own generally means “teaches” (from uczyć kogoś – to teach someone):
- On uczy dzieci. – He teaches children.
uczy się means “learns” (literally “teaches himself/herself”):
- On uczy się. – He learns / he is studying.

So uczy się in the sentence is “(she) learns / is studying,” not “(she) teaches.” The się is obligatory if you want the meaning learn rather than teach.

Both stresuje się and uczy się use się. Is się doing the same thing in both cases?

Grammatically, yes: się is the reflexive particle in both verbs.

stresować się – to get stressed / to be stressed
uczyć się – to learn

In both, się is part of the dictionary form of the verb and must be included. It doesn’t stand for a specific pronoun like myself/himself in a simple way; often it just makes a new, reflexive verb with its own meaning. You normally learn these as full units:

stresować się
uczyć się rather than stresować
- się, uczyć
  - się separately.

Why is it ocenami and not oceny? What case is that?

Ocenami is the instrumental plural of ocena (mark, grade).

The structure stresować się czymś requires the instrumental case:

stresować się egzaminem – to be stressed by the exam
stresować się pracą – to be stressed by work
stresować się ocenami – to get stressed about grades

So:

singular instrumental: oceną
plural instrumental: ocenami

Using nominative oceny instead would be incorrect here, because the verb stresować się governs the instrumental case.

Could you also say stresuje się ocenami as “she is stressed about grades,” or is it more “she gets stressed”?

It can express both, depending on context.

In Polish, the present tense of an imperfective verb like stresować się can describe:

a general tendency / habit:
- My sister tends to get stressed about grades easily.
a situation happening now (with context):
- Right now my sister is stressed about grades.

So Moja siostra szybko stresuje się ocenami can mean:

My sister gets stressed about grades quickly / easily (habit), and also
My sister is (getting) stressed quickly about grades (current situation), but in isolation it most naturally sounds like a general tendency.

What exactly does szybko modify here? Could we move it somewhere else?

Szybko is an adverb meaning quickly, fast, easily in this context. It modifies stresuje się:

Moja siostra szybko stresuje się ocenami
≈ My sister quickly gets stressed about grades / My sister easily gets stressed about grades.

Polish word order is flexible. You can move szybko and się, for example:

Moja siostra szybko się stresuje ocenami.
Moja siostra stresuje się szybko ocenami. (less typical, but possible)

The most natural versions are:

Moja siostra szybko się stresuje ocenami.
Moja siostra szybko stresuje się ocenami.

The meaning stays the same; only the emphasis can shift slightly.

What does coraz lepiej mean exactly, and why is it lepiej and not lepsza or bardziej dobrze?

Coraz lepiej literally is better and better / more and more better.

dobrze – well (adverb)
lepiej – better (adverb, comparative of dobrze)

The pattern is:

coraz
- comparative = more and more + comparative
  - coraz szybciej – faster and faster
  - coraz później – later and later
  - coraz lepiej – better and better

We use the adverb lepiej because it modifies the verb uczy się (how she learns – she learns better).
Lepsza is an adjective (better describing a noun: a better book, a better grade), so it doesn’t fit here.
Bardziej dobrze is ungrammatical; you normally use lepiej as the comparative of dobrze.

Why is there a comma before ale?

In Polish, you normally put a comma before ale (but) when it connects two clauses or contrasting parts:

…ocenami, ale uczy się coraz lepiej.

Each side has its own verb:

(ona) stresuje się
(ona) uczy się

Because there are two clauses, you must separate them with a comma before ale. This is a standard rule in Polish punctuation.

Can I reverse the order of the clauses: Moja siostra uczy się coraz lepiej, ale szybko stresuje się ocenami?

Yes, that is grammatical and natural:

Moja siostra uczy się coraz lepiej, ale szybko stresuje się ocenami.

The meaning is the same; you just change the focus. The original sentence starts by highlighting how easily she gets stressed, then contrasts that with the good news that she’s learning better and better. The reversed version foregrounds her improving learning first, then adds the “but she still gets stressed easily” part.

Why is it moja siostra and not mój siostra?

Polish possessive adjectives agree in gender, number and case with the noun they modify.

siostra (sister) is feminine, singular, nominative.
The corresponding form of mój (my) for feminine singular nominative is moja.

So:

mój – masculine (e.g. mój brat – my brother)
moja – feminine (e.g. moja siostra – my sister)
moje – neuter or non‑personal plural (e.g. moje dziecko, moje książki)

Why is it siostra (not siostrą) and ocenami (not oceny)? How do the cases work here?

Two different cases appear:

siostra – nominative singular
- Subject of the verb: Moja siostra stresuje się… / uczy się…
- The subject in Polish is in the nominative case, so siostra, not siostrą.
ocenami – instrumental plural
- Required by stresować się czymś – to be stressed by/over something.
- Therefore ocenami (instrumental plural), not nominative oceny.

So the pattern is:

subject (nominative): Moja siostra
verb: stresuje się
complement in instrumental: ocenami

Could I say Moja siostra szybko stresuje się z powodu ocen instead of ocenami? Is there a difference?

Yes, you can say:

Moja siostra szybko stresuje się z powodu ocen.
= My sister quickly gets stressed because of grades.

Differences:

stresuje się ocenami – more compact, idiomatic, using instrumental; means “she gets stressed by/over grades.”
stresuje się z powodu ocen – more explicit “because of,” slightly more formal or explanatory.

Both are correct, but stresuje się ocenami is shorter and very natural in everyday speech.

Does uczy się here mean “learns in general” or “studies (for school)”?

In context with ocenami (grades), uczy się most naturally means:

she studies (for school / subjects), i.e. she is doing her schoolwork or studying more and more effectively.

Without any object, uczyć się is understood as learning/studying in general.
If you want to specify what, you add a noun in the genitive:

uczyć się matematyki – to study math
uczyć się polskiego – to learn Polish

Here, the sentence doesn’t specify the subject; it just says her studying/learning is improving.

Is there a difference between szybko stresuje się ocenami and something like jest zestresowana ocenami?

Yes, there is a nuance:

szybko stresuje się ocenami
- Focus on the tendency / process:
- She quickly/easily gets stressed about grades.
- Implies this happens often or easily.
jest zestresowana ocenami
- Focus on the current state:
- She is stressed (right now) because of grades.
- Sounds more like a description of her current condition.

Both are correct, but the original sentence emphasizes her habit of getting stressed easily, contrasted with the fact that she is learning better and better.