Am Abend ziehe ich die Gardine zu, damit es im Zimmer dunkler wird.

Am Abend means in the evening.

• am = an dem
• With parts of the day, German often uses this kind of phrase:
  • am Morgen = in the morning
  • am Nachmittag = in the afternoon
  • am Abend = in the evening

Here it gives the time when the action happens.

A small nuance:

• am Abend can refer to a specific evening or to evening time in general, depending on context.
• abends often means in the evenings / every evening more clearly.

German main clauses follow the verb-second rule.

That means:

• the finite verb must be in the second position
• whatever comes first can be the subject, a time phrase, an object, and so on

So in this sentence:

• Am Abend is in first position
• ziehe must come second
• ich comes after the verb

Basic order:

• Ich ziehe am Abend die Gardine zu. If you move Am Abend to the front:
• Am Abend ziehe ich die Gardine zu.

This is very normal in German.

Because the verb is zuziehen, which is a separable verb.

• infinitive: zuziehen
• present tense in a main clause: ich ziehe ... zu

So:

• ich ziehe die Gardine zu

The verb is split into:

• ziehe = the conjugated part
• zu = the separable prefix, which goes to the end of the clause

Compare:

• Ich mache das Fenster auf. = I open the window.
• Ich ziehe die Gardine zu. = I pull the curtain closed.

In subordinate clauses or infinitive forms, the parts stay together:

• ..., weil ich die Gardine zuziehe
• die Gardine zuzuziehen

Die Gardine is the direct object, so it is in the accusative case.

The noun Gardine is feminine:

• nominative: die Gardine
• accusative: die Gardine

So the article does not change here.

Compare:

• masculine: der Vorhang → den Vorhang
• feminine: die Gardine → die Gardine
• neuter: das Fenster → das Fenster

So die Gardine is accusative, but it looks the same as nominative because the noun is feminine.

They are similar, but not always identical.

• Gardine often means a curtain, especially a lighter or more decorative one, sometimes a net curtain/sheer curtain.
• Vorhang is a more general word for curtain, and can also mean a heavier curtain or drape.

In everyday German, people may use these words a bit loosely, but the distinction above is common.

So die Gardine zuziehen means something like:

• pull the curtain shut
• draw the curtain closed

Here damit means so that and introduces a purpose clause.

The structure is:

• main action: Am Abend ziehe ich die Gardine zu
• purpose: damit es im Zimmer dunkler wird

So the meaning is:

• I draw the curtain in the evening so that the room gets darker.

A useful comparison:

• damit = so that
• um ... zu = in order to

Why use damit here instead of um ... zu?

Because um ... zu usually works when the subject is the same in both parts:

• Ich lerne, um die Prüfung zu bestehen.

But here the second clause has es as its subject:

• damit es im Zimmer dunkler wird

So damit is the natural choice.

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

So:

• main clause: Ich ziehe die Gardine zu.
• subordinate clause: ..., damit es im Zimmer dunkler wird.

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

Other examples:

• ..., weil ich müde bin
• ..., dass er heute kommt
• ..., wenn es regnet

In all of these, the finite verb comes at the end of the subordinate clause.

Here es is similar to English it in expressions like it gets darker.

It does not refer to one specific noun like the curtain. Instead, it is an impersonal subject used for general conditions or states.

So:

• es wird dunkel = it gets dark
• es wird dunkler = it gets darker

In this sentence, the fuller idea is:

• it becomes darker in the room

German often uses es this way.

Dunkler is the comparative form of dunkel.

• dunkel = dark
• dunkler = darker

The sentence is saying that after closing the curtain, the room becomes darker than before.

So:

• damit es im Zimmer dunkler wird = so that it gets darker in the room

If you said damit es im Zimmer dunkel wird, that would mean:

• so that it becomes dark in the room

That is possible grammatically, but it is a slightly different idea. Dunkler focuses on a change in degree, not necessarily total darkness.

Im Zimmer means in the room and describes a location, not movement.

• im = in dem
• in dem Zimmer = in the room

German uses:

• dative for location: im Zimmer
• accusative for movement into somewhere: ins Zimmer = into the room

So here:

• es wird im Zimmer dunkler = it gets darker in the room

There is no movement into the room, so dative is correct.

It could be understood either way, depending on context.

The German present tense can describe:

• a habitual action: Every evening I draw the curtain
• a current or near-present action: In the evening I draw the curtain

Without extra context, many learners will read it as a general or habitual statement.

If German wanted to make the one-time meaning clearer, it might add something like:

• Heute Abend = this evening

If it wanted to stress repetition, it might use:

• Abends = in the evenings

So the sentence as given is natural and slightly open, just like many present-tense sentences in English.