Learn Hardcore German: Ich schließe die Augen, wenn ich schlafen will.

German often uses the definite article (der, die, das) with body parts instead of possessive pronouns (mein, dein, sein), especially when it’s clear whose body part it is.

So:

Ich schließe die Augen. = literally I close the eyes, but understood as I close my eyes.
The subject ich already tells us whose eyes they are, so meine would be redundant.

You can say Ich schließe meine Augen, but it sounds more emphatic or poetic, not the normal everyday phrasing.

What case is die Augen, and why?

Die Augen is in the accusative plural.

Verb: schließen – to close
Ask: Close what? → die Augen
That makes die Augen the direct object, so it must be in the accusative case.

Forms of das Auge (eye):

Nominative plural: die Augen
Accusative plural: die Augen (same form here)
Dative plural: den Augen
Genitive plural: der Augen

Why is the verb order Ich schließe die Augen, wenn ich schlafen will and not … wenn ich will schlafen?

Because wenn introduces a subordinate clause. In German:

In a main clause, the finite verb is in the second position:
Ich schließe die Augen.
(schließe is 2nd idea: Ich – schließe – die Augen)
In a subordinate clause (introduced by wenn, weil, dass, etc.), the finite verb goes to the end:
…, wenn ich schlafen will.

With a modal verb like wollen, the pattern is:

Main clause: Ich will schlafen. (finite will in 2nd position, full verb schlafen at the end)
Subordinate clause: …, wenn ich schlafen will. (subject ich, then full verb schlafen, then finite modal will at the very end)

Why is it wenn and not wann in this sentence?

Wenn and wann both relate to time, but they are used differently.

wenn = when / whenever / if (for conditions or repeated situations)
- Ich schließe die Augen, wenn ich schlafen will.
  → Whenever I want to sleep, I close my eyes.
wann = at what time (a question about a specific time)
- Wann willst du schlafen? – When do you want to sleep?

So you use:

wenn for conditional or repeated situations
wann only in real or indirect questions about the time

Why not use als instead of wenn here?

Als is used mainly for one specific event in the past:

Als ich ein Kind war, habe ich früh geschlafen.
(When I was a child… one time period in the past)

Wenn is used for:

general truths
repeated events
conditions
present and future situations

In Ich schließe die Augen, wenn ich schlafen will, we have a general, repeated situation in the present (“whenever I want to sleep”), so wenn is correct. Als would be wrong here.

Can I change the order of the clauses? For example: Wenn ich schlafen will, schließe ich die Augen.

Yes, you can absolutely invert the order of the clauses:

Ich schließe die Augen, wenn ich schlafen will.
Wenn ich schlafen will, schließe ich die Augen.

Both are correct and mean the same thing.

Rules to note:

The subordinate clause with wenn has the conjugated verb at the end.
After a starting subordinate clause, the following main clause still keeps the verb in the 2nd position:
- Wenn ich schlafen will, schließe ich die Augen.
  (schließe is still in 2nd position of the main clause: schließe – ich – die Augen)

What is the difference between schlafen and einschlafen here?

schlafen = to sleep (the state of sleeping)
einschlafen = to fall asleep (the process of starting to sleep)

In your sentence:

wenn ich schlafen will = when I want to sleep (generally be in a sleeping state)
If you want to focus on the moment of falling asleep, you could say:
Ich schließe die Augen, wenn ich einschlafen will.
→ I close my eyes when I want to fall asleep.

Both are grammatically correct, but they describe slightly different ideas.

Why is it ich schlafen will and not ich zu schlafen will?

Modal verbs in German (like wollen, können, müssen, dürfen, sollen, mögen) connect directly with the infinitive. You do not use zu between a modal and its verb.

Correct:

ich will schlafen
wenn ich schlafen will

Incorrect:

ich will zu schlafen
wenn ich zu schlafen will

The zu + infinitive construction is used with other verbs (e.g., anfangen, versuchen, vergessen) and in some fixed patterns, but not with modal verbs.

What does wollen really mean here? Does will mean future like English “will”?

No. In German:

wollen = to want (to), to intend
It does not form the future tense the way English will does.

So:

ich will schlafen = I want to sleep
wenn ich schlafen will = when I want to sleep

If you want to say I will sleep (as a future statement), in German you normally just use the present tense with a time expression:

Morgen schlafe ich lange. – Tomorrow I will sleep in (sleep a long time).

The auxiliary werden is the usual future marker:

Ich werde schlafen. – I will sleep. (more formal / explicit future)

Is schließe irregular? How is schließen conjugated?

Schließen is mostly regular in the present tense, but it has a vowel change in the past.

Present tense:

ich schließe
du schließt
er/sie/es schließt
wir schließen
ihr schließt
sie/Sie schließen

Important other forms:

Simple past (Präteritum): ich schloss
Past participle: geschlossen
→ Ich habe die Tür geschlossen. – I closed the door.

Why is Augen capitalized, but schlafen and will are not?

German capitalization rules:

All nouns are capitalized:
- die Augen, der Tisch, das Auto
Verbs are not capitalized unless they start a sentence:
- schlafen, will, schließe
Pronouns like ich are lowercase in German (unlike I in English), unless they begin a sentence.

So in Ich schließe die Augen, wenn ich schlafen will.:

Augen = noun → capitalized
schließe, schlafen, will = verbs → lowercase
ich = pronoun → lowercase (here not at the beginning of the sentence)

Could I say Ich mache die Augen zu, wenn ich schlafen will instead of Ich schließe die Augen?

Yes, that is a very natural alternative:

Ich schließe die Augen, wenn ich schlafen will.
Ich mache die Augen zu, wenn ich schlafen will.

Both mean essentially the same: I close my eyes when I want to sleep.

Nuance:

schließen is a bit more neutral/formal.
zumachen / die Augen zumachen is more colloquial, everyday speech.

Grammatically both are fine.