Atak był niespodzianką.

Because atak is a masculine singular noun in Polish.

In the past tense, Polish verbs agree with the gender and number of the subject:

• był = masculine singular
• była = feminine singular
• było = neuter singular
• byli / były = plural

So in Atak był niespodzianką, the subject is atak, so the verb must be był.

Because after być in this kind of sentence, Polish often uses the instrumental case for the noun that describes what something was.

So:

• niespodzianka = nominative, dictionary form
• niespodzianką = instrumental singular

This is very common in Polish:

• On jest nauczycielem. = He is a teacher.
• To była katastrofą. = It was a catastrophe.
• Atak był niespodzianką. = The attack was a surprise.

For many feminine nouns ending in -a, the instrumental singular ends in -ą.

Very often, yes, when a noun tells you what someone or something is/was.

In sentences like this, Polish usually uses:

• X jest Y-em / Y-ą
• X był Y-em / Y-ą

Examples:

• Jan jest lekarzem. = Jan is a doctor.
• To było problemem. = That was a problem.
• Atak był niespodzianką.

However, Polish also has other patterns, especially with to:

• To jest niespodzianka.
• Atak to była niespodzianka.

So instrumental is very common and very natural here, but there are some other constructions too.

Because Polish does not have articles like a/an/the.

So atak can mean:

• attack
• an attack
• the attack

And niespodzianką can correspond to:

• a surprise
• sometimes the surprise, depending on context

You figure out whether English needs a or the from the context, not from a separate Polish word.

Yes. Polish word order is more flexible than English because endings show grammatical roles.

The neutral order here is:

• Atak był niespodzianką.

But you could also say:

• Niespodzianką był atak.

That shifts the emphasis a bit, roughly like:

• What was a surprise was the attack
• or The surprise was the attack

So the original sentence is the most straightforward, but other orders are possible for style or emphasis.

Atak is a noun meaning attack.

It is a masculine inanimate noun. Its basic form is atak.

Some useful forms:

• atak = nominative singular
• ataku = genitive singular
• atakowi = dative singular
• atak = accusative singular
• atakiem = instrumental singular
• ataku = locative singular

In this sentence, atak is the subject, so it appears in the nominative.

A rough pronunciation guide:

• nie-spo-DZYAN-ką

A few key points:

• nie sounds roughly like nye
• dzia sounds like ja in jam, but softer, with a dy / j quality
• ą is a nasal vowel

At the end of a word, ą is often pronounced approximately like on/om, depending on speech and surrounding sounds. So niespodzianką may sound a bit like:

• nyespodzian-kon

But not exactly like normal English on. It is nasalized.

Also, był is pronounced roughly like:

• biw or bew with a short y sound and ł pronounced like English w

Yes. That is also very natural.

• Atak był niespodzianką. = The attack was a surprise.
• Atak był zaskoczeniem. = The attack was surprising / The attack came as a surprise.

The difference is small, but there is a nuance:

• niespodzianka = surprise, often more concrete as a noun
• zaskoczenie = surprise as a reaction, shock, or the state of being surprised

Both work, but niespodzianką matches the English sentence very directly.

Yes. In Polish past tense, the verb form also shows gender and number.

So był tells you:

• it is past tense
• the subject is masculine singular

Compare:

• Atak był niespodzianką. = masculine singular
• Wojna była tragedią. = feminine singular
• Zdarzenie było dziwne. = neuter singular

This is a major difference from English, where was does not show grammatical gender.

Yes, in principle it can, because Polish has no articles.

Without context, atak does not itself tell you whether English should use:

• an attack
• the attack

In real usage, context usually makes it clear. Very often, this sentence will be understood as The attack was a surprise, because it sounds like a specific attack already known in the conversation.

Because the verb agrees with the subject, not with the predicate noun.

Here:

• atak = subject
• niespodzianką = predicate noun in the instrumental

So the verb must match atak, which is masculine:

• Atak był niespodzianką.

Compare:

• Niespodzianka była wielka. = Here niespodzianka is the subject, so the verb is była.

This is similar to English: in The attack was a surprise, was goes with attack, not with surprise.

Yes, it is completely natural.

It is a normal and correct way to say that the attack was unexpected. Depending on style and context, Polish speakers might also say:

• Atak był zaskoczeniem.
• Atak okazał się niespodzianką.
• Atak był nieoczekiwany.

But Atak był niespodzianką is grammatically correct and natural.