To był nieprzewidziany wydatek.

In this sentence, to is a very common Polish word used in identification or explanation sentences, and it often corresponds to English this, that, or it, depending on context.

So:

• To był nieprzewidziany wydatek.
  = That was an unexpected expense.
  = It was an unforeseen expense.

Polish to is more flexible than any one English word, so the exact translation depends on the situation.

Because wydatek is a masculine singular noun, and in this kind of past-tense sentence, Polish often matches the past form of być (to be) to the noun being identified.

• wydatek = masculine
• past tense of być for masculine singular = był

Compare:

• To był problem. = That was a problem.
• To była pomyłka. = That was a mistake.
• To było wydarzenie. = That was an event.

So był is used because wydatek is masculine.

Nieprzewidziany means unforeseen, unexpected, or not anticipated.

It comes from the verb przewidzieć, which means to foresee / to predict / to anticipate.

So:

• przewidziany = foreseen / anticipated
• nieprzewidziany = unforeseen / unanticipated

In this sentence, nieprzewidziany wydatek means an expense that was not planned or expected.

Wydatek means expense, expenditure, or cost.

In everyday English, expense is usually the best translation here.

Examples:

• duży wydatek = a big expense
• niepotrzebny wydatek = an unnecessary expense
• niespodziewany wydatek = an unexpected expense

Plural:

• wydatek = expense
• wydatki = expenses

Because it has to agree with wydatek.

In Polish, adjectives change their endings to match the noun in:

• gender
• number
• case

Here, wydatek is:

• masculine
• singular
• nominative

So the adjective also has the masculine singular nominative form:

• nieprzewidziany wydatek

Compare:

• nieprzewidziany wydatek = masculine
• nieprzewidziana sytuacja = feminine
• nieprzewidziane wydarzenie = neuter

This kind of agreement is one of the core patterns in Polish grammar.

Yes. The past tense is shown by był, which is the past masculine singular form of być (to be).

So:

• jest = is
• był = was

That makes the whole sentence:

• To był nieprzewidziany wydatek.
  = That was an unexpected expense.

If you wanted the present tense, it would be:

• To jest nieprzewidziany wydatek.
  = This/That is an unexpected expense.

Although in natural Polish, present-tense sentences with to are often phrased a bit differently depending on context.

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

So wydatek can mean:

• an expense
• the expense
• sometimes just expense in a general sense

The context tells you which meaning is intended.

That is why:

• To był nieprzewidziany wydatek.

can naturally translate as:

• That was an unexpected expense.

even though there is no separate word for an.

Yes. That would also be natural.

Similar adjectives include:

• nieprzewidziany = unforeseen, unplanned, not anticipated
• niespodziewany = unexpected, surprising
• nieoczekiwany = unexpected, not expected

All of these can work, but the nuance is a little different:

• nieprzewidziany wydatek often sounds like an expense that was not accounted for or planned in advance
• niespodziewany wydatek stresses that it came as a surprise
• nieoczekiwany wydatek is a more general unexpected expense

So the original sentence is a very good choice if the idea is an expense that wasn’t foreseen/planned.

The normal, neutral word order here is:

• To był nieprzewidziany wydatek.

That is the most natural way to say it.

Polish word order is more flexible than English, but changing it usually changes emphasis. For example:

• To był wydatek nieprzewidziany.

This is possible, but it sounds more marked or stylistically special. It may put extra focus on nieprzewidziany.

For a learner, the safest pattern is:

• to był + adjective + noun

A rough English-friendly guide is:

• nyeh-psheh-vee-JAH-ny

A few helpful points:

• nie often sounds like nye
• rz usually sounds like the zh sound in measure
• dzi sounds a bit like jee / dzh with a softer quality
• the stress in Polish is usually on the second-to-last syllable

So in nieprzewidziany, the stress falls on:

• nie-prze-wi-dzia-ny

You do not need perfect pronunciation right away, but it helps to listen for the cluster prze and the stressed dzia.

Yes, very often.

In Polish, to can point to:

• a thing
• an event
• a situation
• something just mentioned

So To był nieprzewidziany wydatek could be said after someone asks why money disappeared, why the bill was higher, or why the budget changed.

In that sense, to is often like English:

• That was...
• It was...

So it does not have to mean a literal object you can point at.

Here wydatek is in the nominative singular.

That is the basic dictionary form of the noun:

• wydatek = nominative singular

It is nominative because this sentence is an identifying statement:

• To był nieprzewidziany wydatek.

The adjective nieprzewidziany is also in nominative singular masculine to match it.

If the noun were used in another function, the case would change. For example:

• Nie mam tego wydatku would be wrong in meaning here, but it shows that Polish nouns change form by case.
• A more realistic example: Nie przewidziałem tego wydatku.
  = I didn’t foresee this expense.

There, wydatku is genitive singular.

So the form wydatek specifically is the nominative singular form.

It is neutral and perfectly natural.

• wydatek is a normal everyday word
• nieprzewidziany is slightly more formal than something very conversational, but still completely standard
• the whole sentence sounds natural in speech and writing

It would fit well in:

• conversation about money
• household budgeting
• business or administrative contexts
• written explanations

So it is not slang, and it is not overly formal either.