Podstawą jest wyciąg bankowy.

Because podstawą is the instrumental singular form of podstawa.

In Polish, after być (to be), a noun that describes what something is is often put in the instrumental case. So:

• podstawa = nominative, dictionary form
• podstawą = instrumental singular

So in Podstawą jest wyciąg bankowy, podstawą means the basis / the foundation / the main supporting document in a predicate role.

A very common learner pattern is:

• X jest Y-em / Y-ą = X is Y

For example:

• Jan jest nauczycielem = Jan is a teacher
• Ona jest studentką = She is a student

This sentence works in the same way.

The noun in the nominative case is usually the subject, so here the subject is wyciąg bankowy.

• wyciąg is nominative singular
• bankowy agrees with wyciąg
• podstawą is instrumental, so it is the predicate noun, not the subject

So structurally, the sentence is basically:

• Wyciąg bankowy = subject
• jest = verb
• podstawą = complement

Even though podstawą comes first, the case ending tells you its grammatical role.

Because Polish word order is flexible, and moving Podstawą to the front changes the focus or emphasis.

Compare:

• Wyciąg bankowy jest podstawą.
  More straightforward, more neutral.

• Podstawą jest wyciąg bankowy.
  Puts emphasis on what the basis is.

This is very natural in Polish. The fronted element often gives the sentence a slightly more formal, explanatory, or contrastive feel.

Yes. That is fully grammatical and probably the more neutral word order for many learners.

The difference is mainly in emphasis:

• Wyciąg bankowy jest podstawą = A bank statement is the basis
• Podstawą jest wyciąg bankowy = The basis is a bank statement

In real Polish, both are possible. The sentence you were given sounds a bit more like a formal explanation or an answer to an implied question such as What is the basis?

Word by word:

• wyciąg = extract, statement, excerpt
• bankowy = bank-related, banking

Together, wyciąg bankowy is the normal Polish phrase for bank statement.

So even though wyciąg on its own can mean several kinds of extract or excerpt, the adjective bankowy makes the meaning specific.

Because Polish often uses an adjective where English uses a noun as a modifier.

English says:

• bank statement
• school bag
• office building

Polish often prefers adjective-based structures:

• wyciąg bankowy
• torba szkolna
• budynek biurowy

A phrase like wyciąg banku would sound like an extract of the bank or an excerpt from the bank, which is not the normal way to say bank statement.

So wyciąg bankowy is the standard expression.

Here they are:

• podstawa — dictionary form; feminine
• być — dictionary form of jest; to be
• wyciąg — dictionary form; masculine inanimate
• bankowy — adjective agreeing with wyciąg

Agreement matters in Polish, so:

• wyciąg is masculine singular
• therefore the adjective is also masculine singular: bankowy

That is why you get wyciąg bankowy, not bankowa or bankowe.

Normally, not in this sentence.

In standard Polish, Podstawą wyciąg bankowy would sound incomplete or ungrammatical.

You need jest:

• Podstawą jest wyciąg bankowy.

There is, however, another common pattern with to:

• Wyciąg bankowy to podstawa.

That version is also natural, but it is a slightly different structure. Learners often compare these:

• X jest Y-em / Y-ą
• X to Y

Both can express X is Y, but they are not always interchangeable in every sentence.

Yes. Podstawa can mean several related things depending on context:

• basis
• foundation
• main support
• essential document / key element

In a practical or administrative context, podstawa often means the thing something is based on, especially a document, rule, or justification.

So in this sentence, podstawą most likely means something like:

• the basis
• the main supporting document
• the document on which something is based

A rough English-friendly guide would be:

pod-STAH-vong yest VIH-chongk ban-KO-vih

A few useful pronunciation notes:

• ą is a nasal vowel; at the end of podstawą, it sounds roughly like -ong
• j in Polish sounds like English y in yes
• wy is not exactly English ვი or wee; the y is a distinct Polish vowel, but vih is a useful rough approximation
• ciąg contains a nasal sound; learners often simplify it too much, but chongk is a decent first approximation
• stress in Polish usually falls on the second-to-last syllable:
  • pod-STA-wą
  • jest
  • wy-CIĄG
  • ban-KO-wy

It sounds fairly neutral to formal, especially because of the word order Podstawą jest...

You might hear it in:

• banking
• administration
• legal or official explanations
• formal written instructions

A less formal or more conversational version might be:

• Wyciąg bankowy to podstawa.
• Podstawą jest bank statement would not be correct Polish-English mixing, of course, but learners sometimes think in that direction

So yes, the given sentence has a somewhat formal, explanatory tone.