To zapewne błąd.

In Polish, the verb to be (jest) is often omitted in the present tense, especially in simple statements.

So:

• To zapewne błąd.
• To jest zapewne błąd.

both mean essentially the same thing: This/That is probably a mistake.

The version without jest is very natural and common.

Here, to means this or that, depending on context.

It is a very common Polish word used in identifying statements, such as:

• To błąd. = This/That is a mistake.
• To mój brat. = This/That is my brother.

So in To zapewne błąd, to points to something and identifies it as a mistake/error.

Zapewne means probably, presumably, or most likely.

It expresses fairly strong probability. In many contexts, it sounds a little more formal or thoughtful than some other words for probability.

So:

• To zapewne błąd. = It is probably a mistake.

It suggests the speaker thinks this is the most likely explanation.

All three can often be translated as probably, but they are not identical.

• chyba — very common in everyday speech; often a bit more casual
• prawdopodobnie — more neutral/formal, literally probably
• zapewne — also probably/presumably, often slightly more literary, careful, or confident in tone

For example:

• To chyba błąd. = It’s probably a mistake.
  Very natural in speech.
• To prawdopodobnie błąd. = It’s probably a mistake.
  Neutral and straightforward.
• To zapewne błąd. = It’s presumably / probably a mistake.
  Slightly more elegant or formal.

The differences are mostly about style and nuance, not basic meaning.

Because this sentence is an identifying statement: This/That is a mistake.

After to in this kind of sentence, Polish commonly uses the nominative case:

• To błąd.
• To problem.
• To katastrofa.

So błąd is nominative singular.

You may also see structures with jest where case usage can vary in other sentence types, but in this very common To ... pattern, nominative is the normal choice.

Yes. That is completely correct.

Compare:

• To zapewne błąd.
• To jest zapewne błąd.

Both are natural. The version without jest is often more concise and idiomatic in simple present-tense definitions or identifications.

Yes, but some orders sound more natural than others.

The most natural versions are:

• To zapewne błąd.
• To jest zapewne błąd.
• To błąd, zapewne. — possible, but more marked and context-dependent

Polish word order is flexible, but changing the order can shift emphasis.

For example:

• Zapewne to błąd. puts more emphasis on probably/presumably
• To zapewne błąd. is the most neutral and natural in many situations

It can mean both mistake and error.

Which English word fits best depends on context:

• in everyday speech: mistake
• in technical, formal, or system-related contexts: error

So To zapewne błąd could mean:

• That’s probably a mistake.
• That’s probably an error.

A rough pronunciation guide:

• b = like English b
• ł = like English w
• ą = a nasal vowel, somewhat like on/om depending on context
• d = like English d

So błąd sounds roughly like bwond or bwont, though the exact nasal sound is Polish-specific.

Also note that ą does not sound like a plain o or a.

It is fairly neutral, but zapewne makes it sound a little more formal, careful, or written than very casual speech.

So:

• To chyba błąd. — more everyday conversational
• To zapewne błąd. — neutral to slightly formal
• To prawdopodobnie błąd. — neutral/formal

None of these is strange, but zapewne has a slightly more polished tone.

Yes. In many sentences, to does not force a strict distinction between this and that the way English often does.

Its exact meaning depends on context:

• something near you: this
• something already mentioned or observed: that
• sometimes even it

So To zapewne błąd can be understood as:

• This is probably a mistake.
• That is probably a mistake.
• It is probably an error.

The situation tells you which English translation works best.