Wenn die Jalousie offen ist, sieht man sofort, dass die neue Glühbirne viel heller ist.

Wenn means when or if, depending on context. Here it introduces a subordinate clause:

Wenn die Jalousie offen ist = When the blind is open

In this sentence, it sounds like a general condition or situation: whenever the blind is open, the result is obvious.

Because it is a subordinate clause, the conjugated verb goes to the end:

• die Jalousie offen ist
• not die Jalousie ist offen

German main clauses usually follow the verb-second rule: the conjugated verb must come in the second position.

The sentence begins with the subordinate clause:

Wenn die Jalousie offen ist, ...

That entire clause counts as position 1. So the main clause must begin with the verb:

• ..., sieht man sofort, ...

This is why you do not say:

• ..., man sieht sofort ... ❌

A useful pattern:

• Wenn ... , sieht man ...
• Weil ... , geht er ...
• Obwohl ... , bleibt sie ...

No. In German, man is an impersonal pronoun meaning things like:

• one
• you
• people
• someone

So sieht man sofort means something like:

• you can immediately see
• one immediately sees
• it is immediately obvious

It does not refer to a male person.

Because dass introduces another subordinate clause. In German subordinate clauses, the conjugated verb normally goes to the end.

So:

• dass die neue Glühbirne viel heller ist
• literally: that the new light bulb much brighter is

This is standard after dass.

Compare:

• Main clause: Die neue Glühbirne ist viel heller.
• Subordinate clause: ..., dass die neue Glühbirne viel heller ist.

Dass means that and introduces a clause explaining what is seen:

• sieht man sofort, dass ...
• you immediately see that ...

So the structure is:

• When the blind is open, you immediately see [that the new light bulb is much brighter].

The dass-clause is the content of what is noticed.

Offen describes the state: open.

• Die Jalousie ist offen. = The blind is open.

Geöffnet is the past participle of öffnen and often emphasizes that something has been opened.

• Die Jalousie ist geöffnet. = The blind has been opened / is opened

In everyday German, for doors, windows, blinds, etc., offen is usually the more natural choice when you simply mean the thing is in an open state.

Die Jalousie usually means a blind or window blind.

Depending on region and context, it can refer to slatted blinds, shutters, or similar window coverings. In this sentence, blind is the most natural English translation.

It is a feminine noun:

• die Jalousie
• eine Jalousie

Glühbirne is a feminine noun, so it takes die in the nominative singular:

• die Glühbirne = the light bulb

The adjective neu must agree with the noun, so after die it becomes:

• die neue Glühbirne

This is normal adjective ending after a definite article in the nominative feminine singular.

Heller is the comparative of hell:

• hell = bright
• heller = brighter

Viel means much or a lot, so:

• viel heller = much brighter

So the sentence says not just that the new bulb is brighter, but that it is noticeably brighter.

Examples:

• Die Lampe ist heller. = The lamp is brighter.
• Die Lampe ist viel heller. = The lamp is much brighter.

German often expresses this idea differently from English. English commonly says:

• you can immediately see it
• it is immediately obvious

German simply says:

• sieht man sofort
• literally: one sees immediately

The idea of it is understood from the following dass-clause:

• ..., sieht man sofort, dass ...

So what is seen is explained afterward.

It is in the nominative case because it is the subject of the clause:

• dass die neue Glühbirne viel heller ist

Here, die neue Glühbirne is the thing that is brighter.

You can spot this from the structure:

• subject: die neue Glühbirne
• predicate adjective: viel heller
• verb: ist

Sofort is an adverb meaning immediately. In this sentence it comes after man:

• sieht man sofort

That is a very natural placement in German. The sentence is structured as:

• verb: sieht
• subject: man
• adverb: sofort
• object/content clause: dass ...

Other word orders may be possible in different contexts for emphasis, but sieht man sofort is the normal, idiomatic choice here.

Yes, depending on context. Wenn can mean both when and if.

Here the sentence could be understood as:

• When the blind is open, you immediately see that the new bulb is much brighter.

But in some contexts, an English speaker might also say:

• If the blind is open, you immediately see that the new bulb is much brighter.

The German does not force a sharp distinction here. It often depends on how general or conditional the speaker wants it to sound in English.

Because there are two different clauses, and each has its own verb:

1. Wenn die Jalousie offen ist

   • verb: ist

2. ..., dass die neue Glühbirne viel heller ist

   • verb: ist

Both clauses describe a state using sein:

• the blind is open
• the new bulb is much brighter

So the repetition is completely normal.