Mechanik powiedział, że jedna opona jest za stara i trzeba ją wymienić.

Polish does not have articles. There is no direct equivalent of English a/an or the.

So Mechanik powiedział... can mean:

• The mechanic said...
• A mechanic said...

Which one is meant depends on context. In a sentence like this, English usually uses the mechanic because the situation is specific.

Powiedział is the past tense of powiedzieć (to say / to tell), and in Polish past tense forms agree with gender.

Here:

• powiedział = masculine singular
• powiedziała = feminine singular
• powiedzieli / powiedziały = plural

Since mechanik is grammatically masculine, the verb is powiedział.

Że means that and introduces a subordinate clause.

So:

• Mechanik powiedział, że... = The mechanic said that...

In English, that is often optional:

• The mechanic said one tire is too old...

In Polish, że is normally used here.

Because opona is a feminine noun, and jeden (one) must agree with it in gender.

So:

• masculine: jeden
• feminine: jedna
• neuter: jedno

Since opona is feminine, you say jedna opona.

This phrase also emphasizes that it is one tire out of several, not all of them.

Adjectives in Polish agree with the noun they describe in gender, number, and case.

Here the noun is:

• opona = feminine singular

So the adjective must also be feminine singular:

• stary = masculine
• stara = feminine
• stare = neuter / non-masculine plural in some contexts

That is why you get jedna opona jest za stara.

In this sentence, za means too.

So:

• stara = old
• za stara = too old

This is a very common Polish pattern:

• za drogi = too expensive
• za mały = too small
• za późno = too late

So opona jest za stara means the tire is too old.

Trzeba is an impersonal word meaning it is necessary, one must, or you have to.

So:

• trzeba wymienić = it needs to be replaced / one must replace it

There is no specific subject here. Polish often uses this impersonal construction when the speaker is talking about necessity in a general way, rather than saying exactly who must do it.

That makes trzeba ją wymienić sound natural and slightly less personal than explicitly saying:

• musisz ją wymienić = you have to replace it
• trzeba ją wymienić = it needs to be replaced

Ją is the accusative singular form of ona (she/it), and here it refers to opona.

Since opona is the direct object of wymienić (to replace), Polish uses the accusative:

• nominative: opona
• accusative: oponę
• pronoun replacing it: ją

So:

• trzeba wymienić oponę = the tire needs to be replaced
• trzeba ją wymienić = it needs to be replaced

A learner may wonder about jej, but jej is not the normal direct-object form here. Ją is the correct choice.

Because trzeba is normally followed by an infinitive.

So the pattern is:

• trzeba + infinitive

Examples:

• trzeba iść = one has to go
• trzeba zapłacić = one has to pay
• trzeba ją wymienić = it has to be replaced

This is one of the most common Polish structures for expressing necessity.

This is about verbal aspect.

• wymienić = perfective
• wymieniać = imperfective

In this sentence, the mechanic means a single completed action: the tire needs to be replaced. That is why wymienić is the natural choice.

Very roughly:

• wymienić = replace it, complete the replacement
• wymieniać = be replacing / replace repeatedly / replace in a general ongoing sense

After trzeba, both aspects are possible in some contexts, but here the perfective wymienić fits best because the result matters: the old tire should be replaced with a new one.

Because short unstressed pronouns like ją often come before the infinitive in natural Polish word order.

So:

• trzeba ją wymienić = the most natural neutral order

You may also hear other orders in special contexts, but they usually sound more emphatic or less neutral.

For a learner, the safest pattern is:

• trzeba + pronoun + infinitive

Examples:

• trzeba go zapytać
• trzeba to zrobić
• trzeba ją wymienić

Yes. You could say:

• Mechanik powiedział, że jedna opona jest za stara i trzeba wymienić oponę.

But this sounds more repetitive, because opona has already been mentioned. Polish, like English, often uses a pronoun instead of repeating the noun:

• ją = it

So trzeba ją wymienić is more natural here.