Sprawdzam numer telefonu jeszcze raz, żeby nie popełnić błędu.

In Polish, the present tense usually covers both:

• I check
• I am checking

So sprawdzam can mean either, depending on context.

Also, sprawdzam is the imperfective verb form, which fits an ongoing action: you are in the process of checking the number.

If you used sprawdzę, that would normally mean I will check or I’ll check.

Polish does not have articles like English the and a/an.

So numer telefonu can mean:

• the phone number
• a phone number

Context tells you which one is meant. In this sentence, it most naturally means the phone number, because the speaker probably has a specific number in mind.

Telefonu is in the genitive singular.

In Polish, one noun often modifies another by putting the second noun in the genitive. So:

• numer telefonu = phone number
• literally: number of the phone

This is a very common structure, similar to:

• klucz do domu = key to the house
• kolor samochodu = the car’s color / color of the car

The base form is telefon, but after numer it becomes telefonu.

By itself, jeszcze can mean things like:

• still
• yet
• more

But the expression jeszcze raz is a fixed phrase meaning:

• again
• once more
• one more time

So here jeszcze raz means the speaker is checking the number again.

Because żeby introduces a subordinate clause.

The part after it explains the purpose of the main action:

• Sprawdzam numer telefonu jeszcze raz = main clause
• żeby nie popełnić błędu = purpose clause

In Polish, subordinate clauses introduced by words like żeby, że, bo, gdy, kiedy are normally separated by a comma.

Here żeby means something like:

• so that
• in order to

It introduces the reason or purpose for the action.

So the structure is:

• Sprawdzam... żeby...
• I’m checking... so that / in order not to...

A close alternative is aby, which is a bit more formal:

• ...aby nie popełnić błędu

Because Polish often uses żeby + infinitive when the subject of both actions is the same.

Here, the person who is checking the number is also the person who wants to avoid making a mistake.

So:

• Sprawdzam numer telefonu jeszcze raz, żeby nie popełnić błędu.

This is natural and means I’m checking the phone number again so as not to make a mistake.

If the subject were different, Polish would usually use a finite verb instead, for example:

• Sprawdzam numer telefonu jeszcze raz, żebyś nie popełnił błędu.
• I’m checking the phone number again so that you don’t make a mistake.

This is about aspect.

• popełnić = perfective
• popełniać = imperfective

The perfective verb popełnić is used because the sentence refers to avoiding a single completed mistake: not to make a mistake.

That is the natural choice here.

If you used popełniać, it would sound more like repeated or habitual mistake-making, which does not fit as well.

Compare:

• nie popełnić błędu = not to make a mistake
• nie popełniać błędów = not to make mistakes / not to keep making mistakes

Because after a negated transitive verb, Polish often uses the genitive instead of the accusative.

Without negation, you would normally say:

• popełnić błąd = to make a mistake

With negation:

• nie popełnić błędu = not to make a mistake

So:

• błąd = accusative form here
• błędu = genitive form here

This is a very important pattern in Polish and often surprises English speakers.

Yes. Polish word order is more flexible than English, because cases show grammatical relationships.

The original sentence is very natural:

• Sprawdzam numer telefonu jeszcze raz, żeby nie popełnić błędu.

But you could also say:

• Jeszcze raz sprawdzam numer telefonu, żeby nie popełnić błędu.
• Sprawdzam jeszcze raz numer telefonu, żeby nie popełnić błędu.

These versions are all understandable. The difference is mostly about emphasis:

• starting with jeszcze raz emphasizes again / once more
• keeping numer telefonu right after the verb sounds very neutral and natural

So the original sentence is a good standard version, but it is not the only possible order.