Learn Hardcore Polish: Gruba zasłona w sypialni sprawia, że w pokoju jest ciemno. - A thick curtain in the bedroom makes the room dark.

w
- locative = being inside a place (no movement)
  - w sypialni = in the bedroom (we are located there)
w
- accusative = movement into (less common, more dynamic)
  - w sypialnię = into the bedroom (going in, not just being there)

Here, the curtain is located in the bedroom, so we use locative:
sypialnia (nom.) → w sypialni (loc.).

w sypialnia is simply ungrammatical; the preposition forces a different case.

Can w sypialni describe the curtain or the place where it is dark?

Yes, and that can be a bit confusing.

The Polish sentence:

Gruba zasłona w sypialni sprawia, że w pokoju jest ciemno.

is normally understood as:

w sypialni = where the curtain is
w pokoju = where it is dark

So:
A thick curtain in the bedroom causes it to be dark in the room.

However, in another context, w sypialni could also be understood as the place where something happens, if the rest of the sentence supports that reading. Polish relies heavily on context and word order for nuance.

What exactly does sprawia mean here, and what form is it?

sprawia is:

3rd person singular
present tense
imperfective aspect
from the verb sprawiać

Basic meanings: to cause, to make, to bring about.

In this sentence:

Gruba zasłona … sprawia, że …
= The thick curtain causes that …
= The thick curtain makes it so that …

It describes an ongoing or general fact, not a one‑time event.

Why is there a comma before że in sprawia, że?

In Polish, że introduces a subordinate clause (like that in English in this kind of sentence).

General rule:
When że introduces a clause, you must put a comma before it.

So:

sprawia, że w pokoju jest ciemno
= causes that it is dark in the room

You cannot write it without the comma in standard Polish, and you cannot normally drop że here either.

Could we say just sprawia, że jest ciemno and omit w pokoju?

Yes, that is grammatically fine:

Gruba zasłona w sypialni sprawia, że jest ciemno.

Then the place where it is dark is understood from context (probably the same bedroom).

Adding w pokoju makes it explicit:

… sprawia, że w pokoju jest ciemno.
= … causes it to be dark in the room.

You might mention w pokoju if:

you want to emphasize that room specifically, or
you are talking about more than one space and need to be precise.

Why do we say w pokoju jest ciemno and not w pokoju jest ciemny or pokój jest ciemny?

Polish often uses adverb‑like forms (ending in ‑o) to describe general states like light, temperature, or weather:

jest ciemno – it is dark
jest jasno – it is bright
jest zimno – it is cold
jest gorąco – it is hot

So:

w pokoju jest ciemno
= it is dark in the room (a general condition inside the room)

If you said:

pokój jest ciemny
= the room is dark (more about a property of the room itself, e.g. dark walls, no windows)

Both can be correct, but jest ciemno focuses on the current lighting, not the inherent character of the room.

Is ciemno an adjective or an adverb here?

Formally, ciemno is an adverb (from the adjective ciemny).

In sentences like:

W pokoju jest ciemno.
Na dworze jest zimno.

it functions as a predicative complement, describing the general state:

it behaves like an adverb
but it fills the slot of what it is like after jest

So you can think of it as adverb used as a predicate:
It is dark / it is darkly → in correct English: it is dark.

Which grammatical cases appear in the sentence, and why?

Sentence: Gruba zasłona w sypialni sprawia, że w pokoju jest ciemno.

Gruba zasłona – nominative singular feminine
- subject of the sentence (the thick curtain)
w sypialni – locative singular feminine
- w
  - locative = in the bedroom
w pokoju – locative singular masculine
- w
  - locative = in the room

No accusative or genitive objects here; the verb sprawia is followed by a że‑clause instead of a noun object.

What is the difference between zasłona and firanka or kurtyna?

These are all types of curtains, but not identical:

zasłona – a thick curtain you can draw to block light or view
firanka – a sheer/net curtain, usually thin, decorative, semi‑transparent
kurtyna – a stage curtain (in a theatre), or sometimes a very large heavy curtain

In this sentence, gruba zasłona strongly suggests a thick, light‑blocking curtain, which fits the idea of making the room dark.

Could we replace sprawia with another verb like powoduje or robi?

Yes, but there are nuances:

powoduje, że – very close in meaning, slightly more formal/technical
- Gruba zasłona … powoduje, że w pokoju jest ciemno.
robi, że – possible but sounds a bit clumsy or colloquial; robi, że is rarely the first choice in standard speech
- Better: robi ciemno w pokoju, but that is also quite colloquial.
czyni, że – very formal, literary, or old‑fashioned.

In everyday neutral Polish, sprawia, że is natural and idiomatic here.