Learn Hardcore Polish: Mój kuzyn żartuje, że bez stresu nie ma dobrych wyników. - My cousin jokes that without stress there are no good results.

Mój kuzyn (who?) → subject, nominative
mojego kuzyna would be genitive or accusative (used after some verbs or prepositions, or to express possession), not correct here.

So:

Mój kuzyn żartuje… = My cousin jokes… (subject in nominative)
Nie znam mojego kuzyna. = I don’t know my cousin. (object in accusative → mojego kuzyna)

Here we need the subject, so nominative: Mój kuzyn.

What is the difference between żartuje and żartuję?

They are different person forms of the same verb żartować (to joke):

żartuję = I joke / I am joking (1st person singular)
żartujesz = you joke (2nd person singular)
żartuje = he / she / it jokes or is joking (3rd person singular)

In the sentence Mój kuzyn żartuje…, the subject is he (my cousin), so we must use żartuje.

Why is there a comma before że and do we always need że here?

Że introduces a subordinate clause, similar to English that in:
My cousin jokes *that without stress there are no good results.*

Polish normally uses a comma before że when it starts a subordinate clause:

Mój kuzyn żartuje, że…

The że cannot be omitted in standard Polish the way English sometimes drops that:

English: He jokes (that) without stress there are no good results.
Polish: On żartuje, że… – you cannot say On żartuje, bez stresu nie ma… in the same way; it sounds wrong or at least unnatural.

Why is it bez stresu and not bez stres?

The preposition bez (without) always takes the genitive case.

stres → nominative (dictionary form)
stresu → genitive (used after bez)

So you must say:

bez stresu = without stress
bez pieniędzy = without money
bez problemu / bez problemów = without (a) problem / without problems

Therefore bez stresu is required by the grammar of the preposition bez.

Why is it nie ma instead of just ma with nie somewhere else?

Ma means has / there is (depending on context), and nie ma is its negative form, often meaning there is no / there are no.

Ma dobre wyniki. = He has good results.
Nie ma dobrych wyników. = He doesn’t have good results. or There are no good results.

In this sentence, the structure is impersonal: bez stresu nie ma dobrych wyników ≈ without stress, there are no good results.

So nie ma functions as a fixed expression for there is no / there are no.

Why are dobrych wyników in that form? Why not dobre wyniki?

Dobre wyniki is nominative plural (used e.g. as subject or direct object in positive sentences):

Są dobre wyniki. = There are good results.

But after nie ma (there is/are no), Polish uses the genitive:

(Są) dobre wyniki. → nominative (positive)
Nie ma dobrych wyników. → genitive plural after negation

So:

wyniki → nominative plural
wyników → genitive plural
dobre → nominative plural, adjective
dobrych → genitive plural, adjective (agrees with wyników)

Because of nie ma, we need genitive: dobrych wyników.

Could we change the word order to bez stresu nie ma wyników dobrych?

In theory, Polish word order is flexible, and Bez stresu nie ma wyników dobrych is grammatically possible, but it sounds unusual or marked.

Normal, neutral order is:

bez stresu nie ma dobrych wyników

Putting dobrych after wyników would sound like you are emphasizing or contrasting good vs. other kinds of results, and it’s still a bit odd. In everyday speech and writing, keep:

dobrych wyników (adjective + noun together).

Why don’t we repeat a pronoun like on (he) before żartuje?

Polish is a pro‑drop language: the personal pronoun is usually omitted because the verb ending already shows the person.

Żartuje. = He/She is joking. (context tells who)
Mój kuzyn żartuje. = My cousin is joking. (subject is explicit)

If you added on here (On, mój kuzyn, żartuje…), it would be either redundant or sound like a special emphasis (e.g. contrasting him with someone else). Normally you don’t need on.

Does kuzyn mean both male and female cousin, like English cousin?

No. Polish distinguishes gender:

kuzyn = male cousin
kuzynka = female cousin

So:

Mój kuzyn żartuje… = My (male) cousin jokes…
Moja kuzynka żartuje… = My (female) cousin jokes…

English cousin is gender‑neutral; Polish is not in this case.

Is żartuje here more like “is joking” (right now) or “jokes” (in general)?

Polish present tense often covers both meanings, and the context decides:

Mój kuzyn żartuje, że… can mean
- My cousin is (right now) joking that…
- My cousin (often) jokes that…

Without extra time markers (like zawsze = always, w tej chwili = at the moment), both readings are possible, just like English can be ambiguous if you don’t specify context.