Die geschlossene Jalousie macht das Zimmer dunkler, obwohl die neue Glühbirne hell ist.

Because geschlossene is being used like an adjective before the noun Jalousie.

• Base verb: schließen = to close
• Past participle: geschlossen = closed
• Used before a noun, it behaves like an adjective: the closed blind

Since Jalousie is feminine singular and has the definite article die, German uses the weak adjective ending -e:

• die geschlossene Jalousie

So this is basically closed turned into an attributive adjective.

For the same grammar reason: both describe a feminine singular noun with the definite article die.

• die geschlossene Jalousie
• die neue Glühbirne

In German, after der / die / das, adjectives usually take weak endings. For feminine nominative singular, that ending is -e.

So:

• die geschlossene Jalousie
• die neue Glühbirne

follow the same pattern.

Here, Jalousie means a window blind or venetian blind.

It does not mean jealousy in German, even though it looks similar to the French/English word. The German word for jealousy is Eifersucht.

So in this sentence:

• die geschlossene Jalousie = the closed blind

Because machen here uses an accusative object:

• etwas macht etwas dunkler
• something makes something darker

So:

• Die geschlossene Jalousie = subject
• macht = verb
• das Zimmer = direct object
• dunkler = result/state

A useful pattern is:

• X macht Y + adjective

Examples:

• Der Regen macht die Straße nass.
• Die Musik macht mich ruhig.
• Die Jalousie macht das Zimmer dunkler.

Also, das Zimmer looks the same in nominative and accusative because Zimmer is neuter:

• nominative: das Zimmer
• accusative: das Zimmer

Because dunkler is a comparative adjective used predicatively, not directly before a noun.

Here it means darker:

• dunkel = dark
• dunkler = darker

In the sentence, it is part of the structure:

• macht das Zimmer dunkler = makes the room darker

When adjectives are used this way, they do not take normal adjective endings like -e, -es, -en, etc.

Compare:

• ein dunkles Zimmer = an adjective before a noun, so it gets an ending
• Das Zimmer ist dunkel. = predicate adjective, no ending
• Die Jalousie macht das Zimmer dunkler. = comparative predicate adjective, no ending

This is one of the most important word-order rules in German.

In the main clause, the conjugated verb goes in second position:

• Die geschlossene Jalousie macht das Zimmer dunkler

In the subordinate clause introduced by obwohl, the conjugated verb goes to the end:

• obwohl die neue Glühbirne hell ist

So the sentence has:

1. Main clause: Die geschlossene Jalousie macht das Zimmer dunkler
2. Subordinate clause: obwohl die neue Glühbirne hell ist

This is very typical German word order.

Obwohl means although or even though. It introduces a contrast.

So the idea is:

• the blind makes the room darker
• although the new light bulb is bright

It shows that the second fact does not prevent the first one.

Grammar point:

• obwohl is a subordinating conjunction
• that is why the verb in its clause goes to the end: hell ist

Because hell often means bright in German.

So:

• hell = bright / light
• dunkel = dark

In this sentence:

• die neue Glühbirne hell ist = the new light bulb is bright

German uses hell very naturally for brightness:

• Das Zimmer ist hell. = The room is bright.
• Die Lampe ist hell. = The lamp is bright.

So even though hell can also mean light-colored, here it clearly means bright.

Because nouns in German have grammatical gender, and Glühbirne happens to be feminine:

• die Glühbirne = the light bulb

There is not always a logical reason you can see from the English meaning; gender is something you usually learn together with the noun.

In this sentence, that feminine gender affects:

• the article: die
• the adjective ending: neue

So:

• die neue Glühbirne

Yes, depending on the exact kind of window covering.

German has several related words:

• die Jalousie = usually a blind, often with slats
• das Rollo = a roller blind / shade
• der Vorhang = curtain

So Jalousie is correct if the object is a blind. If the meaning were specifically a roller blind, Rollo might sound more natural.

The grammar would change with the noun:

• Die geschlossene Jalousie
• Das geschlossene Rollo

Notice that the article changes because Rollo is neuter.

It is singular.

You can tell from:

• the article die, which here is feminine singular
• the noun Jalousie, which is singular

The plural would be:

• die geschlossenen Jalousien

So:

• die geschlossene Jalousie = the closed blind
• die geschlossenen Jalousien = the closed blinds