W sklepie proszę o papierową torbę, bo plastikowa butelka z sokiem jest ciężka.

Because the preposition w meaning in normally requires the locative case when it talks about location.

• sklep = shop/store
• w sklepie = in the shop/store

So here:

• W sklepie = In the store

If w expresses movement into something, Polish often uses the accusative instead, but here the meaning is location, not motion.

Great question. Proszę is one of those very common Polish words with several related uses.

In this sentence, proszę o means:

• I ask for
• I am asking for
• in a shop context, often simply I’d like

So:

• proszę o papierową torbę = I ask for a paper bag

But proszę can also be used as:

• please
• here you are
• go ahead
• yes?

In everyday shopping Polish, people very often say:

• Poproszę papierową torbę. = I’d like a paper bag.

That is often a bit more natural than proszę o papierową torbę, though your sentence is still understandable and grammatical.

After the preposition o in the meaning for / about, Polish often uses the accusative case.

So:

• torba = bag
• papierowa torba = paper bag
• o papierową torbę = for a paper bag

The noun and adjective both change because they are in the accusative:

• papierowa → papierową
• torba → torbę

This is a very common pattern:

• prosić o coś = to ask for something

Examples:

• Proszę o kawę. = I’m asking for coffee.
• Proszę o pomoc. = I’m asking for help.

Because they have different jobs in the sentence.

1. papierową torbę

This is the thing being requested after proszę o, so it is in the accusative.

• nominative: papierowa torba
• accusative: papierową torbę

2. plastikowa butelka

This is the subject of the clause plastikowa butelka z sokiem jest ciężka.

Subjects are normally in the nominative case, so it stays:

• plastikowa butelka

So the contrast is:

• I ask for what? → accusative
• What is heavy? → nominative

Because the preposition z meaning with takes the instrumental case.

So:

• sok = juice
• instrumental singular: sokiem
• z sokiem = with juice

That gives:

• butelka z sokiem = a bottle with juice

This describes the bottle as containing juice.

A useful comparison:

• butelka z sokiem = a bottle with juice
• butelka soku = a bottle of juice

Both can be natural, but they are structured differently:

• z sokiem = using the preposition z
  • instrumental
• soku = genitive, meaning of juice

Because it agrees with butelka, which is a feminine noun.

• butelka = bottle, feminine
• ciężki = heavy, dictionary/base form
• feminine singular form = ciężka

So:

• butelka jest ciężka = the bottle is heavy

In Polish, adjectives must agree with the noun in:

• gender
• number
• case

That is why you see:

• plastikowa butelka
• papierową torbę
• ciężka

All of those adjective forms match the nouns they describe.

Because Polish often leaves out subject pronouns when they are already clear from the verb form.

• proszę already tells you the subject is I
• so ja is unnecessary

Compare:

• (Ja) proszę o papierową torbę.
• I am asking for a paper bag.

In Polish, adding ja is usually only for emphasis, contrast, or clarity.

So the normal, natural version is simply:

• proszę o papierową torbę

Yes, bo means because.

In this sentence:

• bo plastikowa butelka z sokiem jest ciężka
• because the plastic bottle with juice is heavy

Bo is very common in everyday spoken and written Polish. It is natural and conversational.

A more formal alternative is:

• ponieważ = because / since

So:

• bo = common, everyday
• ponieważ = a bit more formal

Polish word order is more flexible than English word order because cases show grammatical function. So yes, some parts can move.

Your sentence:

• W sklepie proszę o papierową torbę, bo plastikowa butelka z sokiem jest ciężka.

Other possible versions include:

• Proszę o papierową torbę w sklepie, bo plastikowa butelka z sokiem jest ciężka.
• Bo plastikowa butelka z sokiem jest ciężka, proszę o papierową torbę.

However, not every version sounds equally natural in every context. The original sounds fine and clear.

A good rule:

• Polish allows movement for emphasis, focus, or style
• but the most neutral order is still usually the easiest for learners

Because papierowy is the normal adjective meaning paper or made of paper.

Its forms include:

• masculine: papierowy
• feminine nominative: papierowa
• feminine accusative: papierową

Since torba is feminine and here it is accusative, we get:

• papierową torbę

So the form is doing two jobs at once:

• matching the noun’s gender: feminine
• matching the noun’s case: accusative

Yes. Torba is a standard word for bag, including a shopping bag.

So:

• papierowa torba = paper bag
• plastikowa torba = plastic bag

You may also hear:

• torebka = small bag, handbag, purse, or sometimes a small bag depending on context

In a store, if you want a shopping bag, torba is the safest basic word.

Yes. A very common and natural store phrase would be:

• Poproszę papierową torbę, bo ta plastikowa butelka z sokiem jest ciężka.

Why this may sound more natural:

• poproszę is very common when ordering or requesting something
• ta plastikowa butelka can sound more specific: this plastic bottle
• in real life, the speaker often points to the item

But your original sentence is still useful for learning, because it clearly shows several important grammar patterns:

• w + locative
• prosić o + accusative
• adjective agreement
• z + instrumental
• predicate adjective agreement in jest ciężka