Mój wujek mówi, że wiek nie jest ważny, jeśli człowiek czuje się młodszy, niż wygląda.

Because wujek is a masculine singular noun in the nominative case, and the possessive adjective has to match it.

So:

• mój = my, for masculine singular nouns in nominative
• moja = my, for feminine singular nouns in nominative
• moje = my, for neuter singular nouns in nominative

Examples:

• mój wujek = my uncle
• moja ciocia = my aunt
• moje dziecko = my child

In this sentence, wujek is the subject, so nominative is the correct case.

Yes, wujek is the common everyday word for uncle.

In modern Polish, people usually use:

• wujek = uncle
• ciocia = aunt

Historically, Polish had more specific family terms, and you may still come across them, but in normal conversation wujek is the standard, natural word.

Also, wujek is a slightly familiar, everyday form. The more formal/basic dictionary form is still wujek in common usage, so this is not unusual at all.

In Polish, a comma is normally used before subordinating conjunctions such as:

• że = that
• jeśli = if
• niż = than

So in this sentence:

• Mój wujek mówi, że...
• ...ważny, jeśli...
• ...młodszy, niż...

all those commas are standard Polish punctuation.

This is one thing English speakers often notice, because Polish uses commas more systematically before these clause-introducing words than English does.

Że means that and introduces a subordinate clause.

So:

• Mój wujek mówi = My uncle says
• że wiek nie jest ważny = that age is not important

Very common pattern:

• myślę, że... = I think that...
• wiem, że... = I know that...
• mówi, że... = he/she says that...

In English, that is often omitted:

• My uncle says age is not important

But in Polish, że is normally expressed.

Because wiek is the subject of the clause wiek nie jest ważny, so it stays in the nominative case.

Breakdown:

• wiek = age
• nie jest ważny = is not important

Since age is the thing being described, nominative is correct.

You would see wieku in other contexts, for example:

• nie mam wieku — actually unnatural
• w tym wieku = at this age
• z wiekiem = with age / as one gets older

So wiek here is simply the base subject form.

Because ważny has to agree with wiek, which is masculine singular.

Agreement in Polish means the adjective matches the noun in gender, number, and often case.

So:

• ważny = masculine singular
• ważna = feminine singular
• ważne = neuter singular (and also some plural contexts)

Examples:

• wiek jest ważny = age is important
• sprawa jest ważna = the matter is important
• dziecko jest ważne = the child is important

Even though in English we say important in the same form every time, in Polish the adjective changes.

Polish can express this idea in different ways. Here, nie jest ważny literally means is not important, which sounds completely natural.

You could also say things like:

• wiek nie ma znaczenia = age doesn't matter
• wiek nie jest istotny = age is not significant
• wiek nie jest ważny = age is not important

So the sentence uses a straightforward, common structure.

Jeśli means if.

So:

• jeśli człowiek czuje się młodszy = if a person feels younger

This is the normal conditional conjunction.

You may also see:

• gdy = when / if
• kiedy = when
• jeżeli = if

Of these, jeśli and jeżeli are very close in meaning. Jeżeli can sound a little more formal or careful, but both are common.

So this sentence could also be:

• ...ważny, jeżeli człowiek czuje się młodszy...

without changing the meaning much.

Here człowiek means person / human being / one, not specifically man.

That is a very common thing for learners to notice, because the literal dictionary meaning can seem misleading.

In this sentence:

• jeśli człowiek czuje się młodszy = if a person feels younger

It is a generic use, similar to English:

• if a person feels younger
• if you feel younger
• if one feels younger

Polish often uses człowiek in this broad, general way.

Because Polish has no articles like English a/an/the.

So:

• człowiek can mean a person, the person, or person/man/human depending on context

Here it is understood as a general statement:

• if a person feels younger...

You always have to get article-like meaning from context in Polish.

Czuje się means feels in the sense of feels oneself / feels in a certain state.

The verb is:

• czuć = to feel
• czuć się = to feel, to feel oneself, to feel in some condition

Examples:

• Czuję się dobrze. = I feel good.
• Czuje się zmęczony. = He feels tired.
• Czujemy się świetnie. = We feel great.

So in the sentence:

• człowiek czuje się młodszy = a person feels younger

The się is an important part of this meaning.

Because with this meaning, the verb is czuć się, not just czuć.

Compare:

• czuć coś = to feel something
  
  • Czuję ból. = I feel pain.
• czuć się jakoś = to feel some way
  
  • Czuję się dobrze. = I feel good.

So:

• czuje się młodszy = feels younger
• not czuje młodszy

For English speakers, this is worth memorizing as a whole pattern:

• czuć się + adjective

Yes, się often has flexible placement in Polish, but some positions are more natural than others.

In this sentence:

• człowiek czuje się młodszy

is the normal, natural order.

You may also see się in other positions in different sentences, for example:

• Mój wujek się śmieje.
• Mój wujek śmieje się.

Both can be possible depending on style and rhythm.

But beginners should usually learn the most standard pattern first:

• czuć się
• bać się
• uczyć się
• nazywać się

So here, czuje się młodszy is the best model to copy.

Because młodszy is the comparative form: younger.

• młody = young
• młodszy = younger

The sentence says:

• czuje się młodszy, niż wygląda
• feels younger than he looks

So a comparison is being made between:

1. how a person feels
2. how a person looks

That is why the comparative form is required.

It agrees with człowiek, which is masculine singular.

So:

• człowiek czuje się młodszy = a person feels younger

If the subject were feminine, the form would change:

• kobieta czuje się młodsza = a woman feels younger

If the subject were neuter:

• dziecko czuje się młodsze = a child feels younger

This is another example of adjective agreement.

Niż means than and is used in comparisons.

So:

• młodszy, niż wygląda = younger than he/she looks

Very common pattern:

• większy niż... = bigger than...
• lepszy niż... = better than...
• starszy niż... = older than...

This is one of the standard ways to build comparisons in Polish.

Because niż here introduces a subordinate comparative clause:

• niż wygląda = than he/she looks

In Polish punctuation, that comma is standard.

English often does not force punctuation in quite the same way, so this can feel unusual to English-speaking learners, but in Polish it is normal.

Wygląda means looks / appears.

It is the 3rd person singular form of wyglądać:

• wyglądam = I look
• wyglądasz = you look
• wygląda = he/she/it looks

So:

• niż wygląda = than he/she looks

In this sentence, the subject is understood from context as the same generic person mentioned earlier: człowiek.

Because Polish usually does not need subject pronouns when the verb ending already shows the person and number.

Here:

• wygląda = he/she/it looks

So Polish does not need to add on / ona / ono unless there is some special emphasis or contrast.

This is very common in Polish:

• Mówię po polsku. = I speak Polish.
• Idziemy do domu. = We are going home.
• Wygląda młodo. = He/she looks young.

Grammatically, wygląda could mean:

• he looks
• she looks
• it looks

But in this sentence it refers back to the generic człowiek, so the idea is:

• if a person feels younger than they look

In natural English, they look is often the best translation, because the Polish sentence is generic rather than specifically about a male person.

Yes, Polish word order is more flexible than English, although not every version sounds equally natural.

The given sentence:

• Mój wujek mówi, że wiek nie jest ważny, jeśli człowiek czuje się młodszy, niż wygląda.

is neutral and natural.

You could rearrange parts for emphasis, for example:

• Mój wujek mówi, że jeśli człowiek czuje się młodszy, niż wygląda, wiek nie jest ważny.

This still makes sense, but the emphasis changes slightly: it foregrounds the condition first.

So the original version is a very good standard model.

Yes, mówi is present tense:

• mówi = says / is saying

In both Polish and English, present tense is often used for general opinions, repeated statements, or things someone says as a principle.

So:

• Mój wujek mówi... can mean
  
  • My uncle says...
  • My uncle always says...
    depending on context

It does not have to mean only that he is speaking at this exact moment.

No. The adjective is agreeing with the actual noun wiek, not with an implied English-style it.

So:

• wiek is masculine singular
• therefore ważny must also be masculine singular

Polish agreement follows the noun actually present in the sentence.

A more direct version might use you:

• Mój wujek mówi, że wiek nie jest ważny, jeśli czujesz się młodziej, niż wyglądasz.

That means:

• My uncle says age isn’t important if you feel younger than you look.

Notice that this version changes the grammar:

• czujesz się = you feel
• młodziej instead of młodszy
• wyglądasz = you look

The original sentence is more general and impersonal, which is why it uses człowiek.

Good question. Polish can describe feeling younger in more than one way.

Here we have:

• czuć się młodszy = to feel younger

This uses an adjective, agreeing with the subject.

But Polish also sometimes uses an adverb:

• czuć się młodziej = to feel younger

Both can occur, though czuć się młodszy/młodsza is a very natural, concrete pattern when the subject is clearly a person.

So the sentence’s version is fully correct and very normal.

This one sentence gives you several very useful patterns:

• mówi, że... = says that...
• X nie jest ważny/a/e = X is not important
• jeśli... = if...
• człowiek used generically = a person / one
• czuć się + adjective = to feel + adjective
• comparative + niż... = comparative + than...
• omission of subject pronouns, because verb endings carry information
• adjective agreement with gender and number

So it is a very rich example sentence for learners.