W kiosku obok dworca kupiłam gazetę i wodę, płacąc jedną monetą i małym banknotem.

Polish often drops subject pronouns when they are not needed. The verb already shows the person, and in the past tense it also shows gender.

So kupiłam already means I bought and tells us the speaker is female. Because of that, ja is unnecessary unless you want extra emphasis, as in Ja kupiłam, nie on.

This is because Polish past tense agrees with the speaker’s gender.

• kupiłam = I bought said by a woman
• kupiłem = I bought said by a man

Both forms are first person singular, but the ending changes depending on whether the speaker is female or male.

The preposition w means in or inside, and when it describes location, it requires the locative case.

The noun kiosk therefore changes to kiosku:

• nominative: kiosk
• locative: w kiosku

So w kiosku means in the kiosk / at the newsstand.

The preposition obok means next to / beside, and it requires the genitive case.

So:

• nominative: dworzec
• genitive: dworca

That is why Polish says obok dworca.

Because they are the direct objects of kupiłam. The verb kupić normally takes the accusative case.

For many feminine singular nouns ending in -a, the accusative singular ends in -ę:

• gazeta → gazetę
• woda → wodę

So kupiłam gazetę i wodę means I bought a newspaper and water.

Płacąc is a contemporary adverbial participle. In English, it often corresponds to while paying or sometimes by paying.

So the phrase:

płacąc jedną monetą i małym banknotem

means something like:

• while paying with one coin and a small banknote
• or paying with one coin and a small banknote

A very important rule: the participle must refer to the same subject as the main verb. Here, the person who bought is also the person who was paying.

In Polish, adverbial participial phrases like płacąc... are normally separated by commas.

So the comma marks the extra accompanying action:

• main action: kupiłam gazetę i wodę
• accompanying action: płacąc jedną monetą i małym banknotem

This is standard punctuation in Polish.

Because after płacić, Polish often uses the instrumental case to show the means used for paying.

So:

• moneta → monetą
• banknot → banknotem

This is similar to other Polish expressions such as:

• płacić kartą = to pay by card
• płacić gotówką = to pay in cash

Here it means the speaker paid with one coin and a small banknote.

Because adjectives and adjective-like words must agree with the noun in gender, number, and case.

Here both nouns are singular instrumental, but they have different genders:

• moneta is feminine → jedną monetą
• banknot is masculine → małym banknotem

So the forms are different because the nouns they describe are different.

Yes, jedną literally means one in the feminine instrumental singular. In this sentence it emphasizes that the speaker used a single coin.

So jedną monetą is not just with a coin, but specifically with one coin.

Polish uses numeral forms that also match the noun’s gender and case, which is why the form is jedną, not jeden.

This is about aspect, which is very important in Polish.

• kupić is perfective: it presents the purchase as a completed action
• płacić is imperfective: it presents paying as an ongoing or accompanying action

The participle -ąc is formed from imperfective verbs, so płacąc comes from płacić, not from perfective zapłacić or another completed form.

That combination is very natural:

• completed main event: kupiłam
• simultaneous accompanying activity: płacąc...

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

This sentence is natural as written, but other orders are possible if you want different emphasis, for example:

• Gazetę i wodę kupiłam w kiosku obok dworca, płacąc jedną monetą i małym banknotem.
• W kiosku obok dworca kupiłam wodę i gazetę, płacąc jedną monetą i małym banknotem.

The original version sounds neutral and natural. Changing the order usually changes emphasis, not the basic meaning.