Abends lasse ich den Rollladen unten, damit das Schlafzimmer dunkel bleibt.

Because German main clauses follow the verb-second rule.

In this sentence, abends is placed first for emphasis or style. Once that happens, the conjugated verb must still stay in position 2:

• Abends lasse ich den Rollladen unten.
• Ich lasse abends den Rollladen unten.

Both are correct. The first one highlights in the evenings a bit more.

Abends means in the evening / in the evenings.

The -s is part of a common adverb form used with times of day:

• morgens = in the morning(s)
• mittags = at noon / midday
• nachmittags = in the afternoon(s)
• abends = in the evening(s)
• nachts = at night

So here abends is an adverb, not a noun phrase.

A Rollladen is usually a roller shutter, often the solid shutter on the outside of a window that can be pulled down.

It is not exactly the same as:

• der Vorhang = curtain
• die Jalousie = Venetian blind / slatted blind
• das Rollo = roller blind

So Rollladen usually suggests something sturdier and better at blocking light.

Because Rollladen is the direct object of lasse, so it is in the accusative case.

Rollladen is masculine:

• nominative: der Rollladen
• accusative: den Rollladen

Here, the subject is ich, and the thing being left in a certain position is den Rollladen.

Here lassen means to leave something in a certain state or position.

So:

• den Rollladen unten lassen = to leave the shutter down

The word unten means down or at the bottom. In this sentence it describes the shutter’s position.

Compare:

• Ich lasse das Fenster offen. = I leave the window open.
• Ich lasse die Tür zu. = I leave the door closed.
• Ich lasse den Rollladen unten. = I leave the shutter down.

So the pattern is often:

lassen + object + state/position

Unten describes a position: down / at the bottom.
Runter usually suggests movement downward.

Here the sentence is about the shutter’s final state, not the action of moving it:

• Ich lasse den Rollladen unten. = I leave the shutter down.
• Ich mache den Rollladen runter. = I pull the shutter down.

So unten fits better because the sentence focuses on the shutter being in the down position.

Damit means so that or in order that.

It introduces a purpose clause: it explains the reason or goal of the action in the main clause.

So the structure is:

• main clause: Abends lasse ich den Rollladen unten
• purpose clause: damit das Schlafzimmer dunkel bleibt

In other words: I do this so that the bedroom stays dark.

Because damit introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

So:

• ..., damit das Schlafzimmer dunkel bleibt.

This is normal word order after words like:

• weil = because
• dass = that
• wenn = if/when
• damit = so that

Bleibt means stays or remains, so it emphasizes that the bedroom continues to be dark.

• dunkel ist = is dark
• dunkel bleibt = stays/remains dark

In this sentence, bleibt is a very natural choice because lowering the shutter helps keep the room dark over time.

If you said damit das Schlafzimmer dunkel ist, it would sound less natural in this context, because the idea is not just that it is dark at one moment, but that it remains dark.

Because das Schlafzimmer is the subject of the subordinate clause:

• das Schlafzimmer bleibt dunkel = the bedroom stays dark

If you said im Schlafzimmer, that would mean in the bedroom, which would create a different structure.

For example:

• ..., damit es im Schlafzimmer dunkel bleibt. = so that it stays dark in the bedroom

That is also possible, but it is a different sentence pattern.

Yes. Both are possible, but they are used a little differently.

• Abends = in the evenings / in the evening generally, often habitual
• am Abend = in the evening, often referring to a specific evening or a more neutral time expression

So:

• Abends lasse ich den Rollladen unten.
  = This is something I generally do in the evenings.

• Am Abend lasse ich den Rollladen unten.
  = Also possible, but slightly less like a general routine.

Here abends works especially well because it suggests a regular habit.

They are similar because both can express purpose, but they are not used in exactly the same way.

Use um ... zu when the subject is the same in both parts:

• Ich schließe das Fenster, um das Zimmer warm zu halten.

Use damit when you want a full clause with its own subject:

• Ich lasse den Rollladen unten, damit das Schlafzimmer dunkel bleibt.

Here das Schlafzimmer is the subject of the subordinate clause, so damit is the natural choice.

Yes, absolutely.

• ..., damit das Schlafzimmer dunkel bleibt
• ..., damit es im Schlafzimmer dunkel bleibt

Both are correct.

The version with das Schlafzimmer treats the room itself as the thing that stays dark.
The version with es im Schlafzimmer is a little more like English: so that it stays dark in the bedroom.

The original sentence is idiomatic and completely natural.