Kiedy kończy się benzyna, trzeba szybko znaleźć stację benzynową.

In this sentence, kończyć się is a very common expression meaning to run out or to come to an end.

So:

• kończyć = to finish / to end something
• kończyć się = to finish / to end / to run out

Here, benzyna kończy się means the petrol is running out.

The się does not always mean a truly reflexive idea like oneself. In many Polish verbs, it is just part of the normal dictionary form and changes the meaning of the verb.

For example:

• kończyć coś = to finish something
• coś się kończy = something is ending / running out

So kończy się benzyna is the natural way to say the petrol is running out.

Both word orders are possible.

• Kończy się benzyna
• Benzyna się kończy

Polish word order is more flexible than English word order. The difference is usually about emphasis, rhythm, or style, not basic meaning.

In this sentence, starting with kończy się benzyna sounds natural after kiedy and puts focus on the situation itself: when the petrol is running out.

So:

• Kiedy kończy się benzyna... = neutral and natural
• Kiedy benzyna się kończy... = also correct

Because benzyna is the subject of the clause kończy się benzyna.

The thing that is running out is benzyna, so it stays in the nominative:

• benzyna = nominative singular

This is different from English, where we often say you run out of petrol. In Polish, the structure is often more like:

• the petrol runs out

So benzyna is not the object of the verb here. It is the thing undergoing the action.

Trzeba means something like:

• it is necessary
• one has to
• you have to
• it is necessary to

It is an impersonal form, which means there is no explicit subject like I, you, or we.

So:

• trzeba szybko znaleźć stację benzynową = you need to quickly find a petrol station / it is necessary to quickly find a petrol station

This is very common in Polish when speaking generally.

Because Polish often leaves the subject unstated, especially in general statements.

English often says:

• you have to find a gas station

But Polish can say:

• trzeba znaleźć stację benzynową

This does not refer only to you personally. It means one must, you have to, or it is necessary to in a general sense.

So the sentence sounds natural without any pronoun.

This is about aspect, which is very important in Polish.

• znajdować = imperfective, to be finding / to find repeatedly / to be in the process of finding
• znaleźć = perfective, to find, with emphasis on achieving the result

After trzeba, Polish often uses the infinitive that best matches the intended meaning. Here, the point is not the process of searching, but the goal: successfully finding a petrol station.

So:

• trzeba szybko znaleźć stację benzynową = you need to quickly find a petrol station

That is why the perfective znaleźć fits well.

Because znaleźć takes a direct object in the accusative case.

The base form is:

• stacja benzynowa = a petrol station (nominative)

But after znaleźć:

• znaleźć stację benzynową = to find a petrol station (accusative)

Since stacja benzynowa is a feminine noun phrase, both the noun and adjective change in the accusative singular:

• stacja → stację
• benzynowa → benzynową

Here kiedy means when, introducing a time clause:

• Kiedy kończy się benzyna... = When the petrol is running out...

Depending on context, Polish kiedy can sometimes feel a bit like whenever, but in this sentence the basic meaning is simply when.

A close alternative would be:

• Gdy kończy się benzyna...

Both are natural.

Because Polish normally uses a comma to separate a subordinate clause from the main clause.

Here:

• Kiedy kończy się benzyna = subordinate time clause
• trzeba szybko znaleźć stację benzynową = main clause

So the comma is required:

• Kiedy kończy się benzyna, trzeba szybko znaleźć stację benzynową.

This is more regular in Polish than in English, where comma usage can be less strict.

Yes. Gdy would also work:

• Gdy kończy się benzyna, trzeba szybko znaleźć stację benzynową.

Both kiedy and gdy can mean when. In many contexts they are interchangeable.

Very roughly:

• kiedy is extremely common and neutral
• gdy can sound a little more formal or literary in some contexts, but is still very normal

So a learner can understand both.

This is another aspect question.

• kończy się benzyna = the petrol is running out
• skończy się benzyna = the petrol will run out / will be completely gone

So:

• kończy się focuses on the process or situation as it is happening
• skończy się focuses on completion

In your sentence, kończy się benzyna suggests the urgent situation of noticing that fuel is running low or running out, which fits naturally with trzeba szybko znaleźć...

Benzyna means petrol in standard British English, and in everyday American English it is often translated as gas or gasoline.

So:

• British English: petrol
• American English: gas / gasoline

Be careful not to confuse it with English benzene, which is a different chemical substance.

In this sentence, stacja benzynowa is the normal term for petrol station / gas station.

Yes, it is grammatical and natural.

It sounds like a general statement meaning something like:

• When you’re running out of gas, you need to quickly find a gas station.

A native speaker might also say similar things such as:

• Kiedy kończy ci się benzyna, trzeba szybko znaleźć stację benzynową.
• Gdy kończy się paliwo, trzeba szybko znaleźć stację.

But the original sentence is perfectly good Polish.