Przy oknie wisi firanka, a na parapecie stoi roślina.

Because przy requires the locative case.

• dictionary form: okno = window
• locative singular: oknie

So:

• przy oknie = by / near the window

This is something you simply have to learn with the preposition: przy + locative.

For the same reason: na can take different cases depending on meaning.

Here it describes a location: the plant is already there, on the windowsill. When na means on/at in a static location, it takes the locative.

• dictionary form: parapet = windowsill
• locative singular: parapecie

Compare:

• na parapecie = on the windowsill
• na stół = onto the table, with movement, would use accusative instead

So in this sentence, na parapecie is locative because it answers where?

Polish very often uses position verbs where English might simply use is.

• wisieć = to hang
• stać = to stand
• leżeć = to lie

So:

• firanka wisi = the curtain hangs
• roślina stoi = the plant stands

Using jest would usually sound less natural here, because Polish prefers to describe the object’s physical position more precisely.

It would be understandable, but it is less natural.

A Polish speaker would normally prefer:

• wisi for something hanging
• stoi for something standing upright

So your version is not exactly wrong, but it sounds more basic and less idiomatic. The original sentence is the natural way to describe the scene.

Because they are the subjects of the verbs, so they are in the nominative case.

• firanka is the thing that hangs
• roślina is the thing that stands

In Polish, word order is more flexible than in English. The subject does not have to come before the verb.

So:

• wisi firanka
• stoi roślina

still have nominative subjects, even though the verb comes first.

Both are possible, but they highlight information differently.

The original order:

• Przy oknie wisi firanka

starts with the location, so it sounds like you are describing a scene: first where, then what is there.

The alternative:

• Firanka wisi przy oknie

starts with the curtain itself, so it puts more focus on the curtain.

Polish word order is flexible, and speakers often move elements around depending on what they want to emphasize.

In this sentence, a links two clauses, and and is often the best English translation. But a is not exactly the same as i.

Very roughly:

• i = simple and
• a = and, but with a slight contrast or shift of focus

Here, a feels natural because the sentence presents one part of the scene and then another:

• by the window hangs a curtain, and on the windowsill stands a plant

It is not a strong contrast like but, just a gentle transition from one detail to another.

This is a useful vocabulary point.

• firanka = a sheer curtain, the thin, often white curtain by a window
• zasłona = a heavier curtain or drape

So in this sentence, firanka specifically suggests the lighter kind of curtain, not just any curtain.

Because Polish has no articles.

English needs words like:

• a
• an
• the

Polish does not. So:

• firanka can mean a curtain or the curtain
• roślina can mean a plant or the plant

The exact meaning depends on context.

Yes, they are 3rd person singular present tense forms:

• wisi = hangs / is hanging
• stoi = stands / is standing

In this sentence they describe a current state, not an action in progress in the English dramatic sense. Polish present tense often covers both ideas:

• it hangs
• it is hanging

So the sentence describes how things are positioned now.

A few sounds may stand out to an English speaker:

• przy: the rz sounds like the s in measure or zh
• oknie: knie is pronounced together, roughly ok-nye
• wisi: the si is a soft sh-like sound, roughly vee-shi
• parapecie: final -cie is soft, roughly -che
• roślina: ś is a soft sh sound
• ł in Polish is usually like English w, though this sentence does not contain it

A rough approximation of the whole sentence is:

Pszy ok-nye vee-shi fee-ran-ka, a na pa-ra-pe-tye sto-ee rosh-lee-na.

That is only approximate, but it can help as a first step.