W nowym mieszkaniu stoi wysoki regał, a przy oknie wisi jasna firanka.

Because w means in here, and when w describes a location, Polish uses the locative case.

• mieszkanie → mieszkaniu
• nowe → nowym to agree with mieszkaniu

So:

• w nowym mieszkaniu = in the new apartment

A useful pattern:

• w domu = in the house
• w pokoju = in the room
• w mieszkaniu = in the apartment

The preposition przy almost always takes the locative case too.

So:

• okno → oknie
• przy oknie = by / next to the window

This is just something to learn with the preposition:

• przy stole = by the table
• przy drzwiach = by the door
• przy oknie = by the window

Polish very often uses position verbs where English would often simply say is.

• stać = to stand
• wisieć = to hang

So:

• regał stoi = the bookcase is standing
• firanka wisi = the curtain is hanging

This sounds very natural in Polish because it tells you how the object is positioned.

Compare:

• W pokoju jest regał. = There is a bookcase in the room.
• W pokoju stoi regał. = There is a bookcase in the room, and it is standing there upright.

Yes, sometimes, but the meaning changes slightly.

• W nowym mieszkaniu jest wysoki regał.
  = There is a tall bookcase in the new apartment.

• W nowym mieszkaniu stoi wysoki regał.
  = A tall bookcase stands in the new apartment.

Using jest focuses more on existence.
Using stoi / wisi focuses more on location and position.

In this sentence, stoi and wisi are more vivid and more idiomatic.

Polish word order is much more flexible than English word order.

The sentence starts with W nowym mieszkaniu because it sets the scene first:

• In the new apartment, there is a tall bookcase...

That is very natural in Polish.

Both are grammatical:

• W nowym mieszkaniu stoi wysoki regał.
• Wysoki regał stoi w nowym mieszkaniu.

But the focus is slightly different:

• W nowym mieszkaniu... emphasizes where
• Wysoki regał... emphasizes what

Here a connects two clauses, but it is not exactly the same as i.

• i = simple and
• a = and, while, whereas, or a gentle contrast / shift

In this sentence:

• W nowym mieszkaniu stoi wysoki regał, a przy oknie wisi jasna firanka.

The speaker is moving from one part of the scene to another:

• there is a bookcase here,
• and by the window there is a curtain.

So a feels very natural because it links two related descriptions while slightly separating them.

Because in standard Polish punctuation, when a joins two clauses, it is normally preceded by a comma.

So:

• ..., a ...

This is different from English, where comma use can be a bit looser in some cases. In Polish, the comma before a is expected here.

Because adjectives in Polish must agree with the noun in gender, number, and case.

Here is what is happening:

• nowym mieszkaniu

  • mieszkanie is neuter singular
  • after w it is in the locative
  • so nowe becomes nowym

• wysoki regał

  • regał is masculine singular
  • it is the subject, so it is in the nominative
  • so wysoki stays wysoki

• jasna firanka

  • firanka is feminine singular
  • it is also in the nominative
  • so jasny becomes jasna

A quick summary:

• masculine nominative: wysoki regał
• feminine nominative: jasna firanka
• neuter locative: w nowym mieszkaniu

They are in the nominative singular, because they are the grammatical subjects of the verbs:

• stoi wysoki regał
• wisi jasna firanka

Even though they come after the verb, they are still the subjects.

That can feel unusual to an English speaker, because English relies much more on word order. Polish relies more on case endings.

Polish has no articles, so there is no direct equivalent of a or the.

That means:

• wysoki regał can mean a tall bookcase or the tall bookcase
• jasna firanka can mean a light-colored curtain or the light-colored curtain

The exact meaning depends on context.

In a sentence like this, English usually uses a because the objects are being introduced for the first time.

Not exactly.

• firanka usually means a net curtain or sheer curtain
• zasłona is a heavier curtain, more like drapes

So jasna firanka suggests a light, thin curtain near the window, which fits the image very well.

The best general translation is by the window or next to the window.

Przy usually means something is right beside or close to something.

So:

• przy oknie = by the window
• koło okna = near the window
• na oknie = on the window / on the windowsill, depending on context
• w oknie = in the window

Here przy oknie tells you the curtain is positioned at the window area, which is the natural expression.

They are:

• mieszkanie = apartment / flat
• nowy = new
• stać = to stand
• wysoki = tall / high
• regał = bookcase / shelving unit
• przy = by / next to
• okno = window
• wisieć = to hang
• jasny = light, bright, pale
• firanka = net curtain / sheer curtain

The forms in the sentence are changed to fit grammar, but these are the forms you would normally look up in a dictionary.