Die eingeschaltete Glühbirne über dem Spiegel ist so hell, dass ich die kaputte Jalousie erst jetzt bemerke.

Both are possible, but they do different jobs.

• die eingeschaltete Glühbirne = the switched-on light bulb
  Here, eingeschaltet is used like an adjective in front of the noun.
• die Glühbirne ist eingeschaltet = the light bulb is switched on
  Here, eingeschaltet is part of the predicate after ist.

German often uses past participles as adjectives before nouns, just like English uses forms such as broken window or closed door.

Because it is being used as an adjective before a feminine noun: die Glühbirne.

In die eingeschaltete Glühbirne:

• Glühbirne is feminine
• it is singular
• it has the definite article die

With a definite article, the adjective usually takes the ending -e in the nominative feminine singular.

So:

• die eingeschaltete Glühbirne
• die kaputte Jalousie

This is standard adjective declension.

For the same reason as eingeschaltete.

Jalousie is feminine, and here it is:

• singular
• accusative
• with the definite article die

For feminine singular with die, the adjective ending is again -e.

So:

• nominative: die kaputte Jalousie
• accusative: die kaputte Jalousie

The form looks the same in both cases for feminine singular.

Because this phrase describes location, not movement.

über is a two-way preposition, so it can take:

• dative for location: over / above
• accusative for movement toward something

Here, the bulb is simply located above the mirror:

• über dem Spiegel = above the mirror

If there were movement, you could get accusative:

• Ich hänge die Lampe über den Spiegel.
  = I hang the lamp above the mirror.

So:

• where? → dative → über dem Spiegel
• to where? → accusative → über den Spiegel

It is dative singular.

The noun is der Spiegel. After über in a location meaning, German uses the dative, so:

• der Spiegel → dem Spiegel

That is why the phrase is über dem Spiegel.

so ... dass means so ... that.

It introduces a result:

• ist so hell, dass ...
• is so bright that ...

So the structure is:

• so
  • adjective/adverb
• dass
  • result clause

Examples:

• Es ist so kalt, dass ich zittere.
  = It is so cold that I’m shivering.
• Sie spricht so schnell, dass ich nichts verstehe.
  = She speaks so fast that I don’t understand anything.

Because it is in a dass-clause, and in German subordinate clauses the conjugated verb usually goes to the end.

Main clause:

• Die Glühbirne ist so hell

Subordinate clause:

• dass ich die kaputte Jalousie erst jetzt bemerke

So this is normal German word order:

• dass
  • subject + other elements + verb at the end

Compare:

• Main clause: Ich bemerke die Jalousie erst jetzt.
• Subordinate clause: ..., dass ich die Jalousie erst jetzt bemerke.

Not exactly.

• sehen = to see
• bemerken = to notice

bemerken suggests that something enters your awareness. You may have been able to see it physically before, but only now do you actually notice it.

So in this sentence, bemerke is a very natural choice: the speaker only now notices the broken blind.

They are related, but not used in the same way.

• bemerken usually means to notice
• merken often means to notice, to realize, or to remember mentally, depending on context

Examples:

• Ich bemerke einen Fehler.
  = I notice a mistake.
• Ich merke, dass etwas nicht stimmt.
  = I realize that something is wrong.
• Merk dir das!
  = Remember that!

In this sentence, bemerken works well because the object is a visible thing: die kaputte Jalousie.

erst jetzt means only now.

It adds the idea that something did not happen earlier, but is happening now for the first time.

So:

• jetzt = now
• erst jetzt = only now / not until now

In this sentence:

• ich bemerke ... jetzt = I notice it now
• ich bemerke ... erst jetzt = I’m only noticing it now

That small word erst adds an important nuance.

It can mean both, depending on context.

• hell = bright
• hell = light-colored / pale

Here, because it describes a light bulb and appears with so hell, it clearly means bright.

Examples:

• Das Zimmer ist hell.
  = The room is bright.
• Sie trägt eine hellblaue Jacke.
  = She is wearing a light-blue jacket.

Because in German, subordinate clauses introduced by conjunctions like dass are separated by a comma.

So:

• ..., dass ich die kaputte Jalousie erst jetzt bemerke.

This comma is required in standard German spelling.

Other common conjunctions that also trigger this kind of comma include:

• weil
• obwohl
• wenn
• als

Yes, die Jalousie is a normal word for a window blind or shutter, especially a slatted blind.

Depending on region and exact type, German may also use other words, but Jalousie is very common.

A few related words:

• die Jalousie = blind
• der Vorhang = curtain
• der Rollladen = roller shutter

So here die kaputte Jalousie means a broken blind, not a curtain.

Not exactly.

• die Glühbirne = light bulb
• die Lampe = lamp or sometimes light

So Glühbirne refers specifically to the bulb itself, while Lampe can refer to the whole lighting fixture or the light more generally.

In everyday speech, people sometimes use Lampe loosely, but Glühbirne is more precise here.

Yes. einschalten means to switch on / turn on, so eingeschaltet can describe many devices.

Examples:

• der eingeschaltete Fernseher = the switched-on television
• das eingeschaltete Radio = the switched-on radio
• die eingeschaltete Heizung = the switched-on heating

For a light bulb, eingeschaltet is completely natural.

Yes. Everything up to ist is the subject, and it forms one long noun phrase:

• Die eingeschaltete Glühbirne = the switched-on light bulb
• über dem Spiegel = above the mirror

Together:

• Die eingeschaltete Glühbirne über dem Spiegel
  = The switched-on light bulb above the mirror

German often builds quite long noun phrases before the verb. The main verb of the sentence is ist.

A stripped-down version would be:

• Die Glühbirne ist hell.
• Ich bemerke die Jalousie.

Then the original sentence adds more information:

• eingeschaltete describes the bulb
• über dem Spiegel tells where it is
• so ... dass connects cause/result
• kaputte describes the blind
• erst jetzt adds the idea of only now

This is a useful way to unpack long German sentences: find the core first, then add the modifiers back in.