Learn Hardcore German: Manchmal kann ich nicht einschlafen, weil ich an den nächsten Tag denke. - Sometimes I cannot fall asleep because I think about the next day.

But kann nicht ich einschlafen is wrong in German word order. The conjugated verb must be second, and the subject normally directly follows it in a neutral statement:

✅ Manchmal kann ich nicht einschlafen.
❌ Manchmal kann nicht ich einschlafen. (sounds very marked / contrastive, like “it’s not me who can’t sleep, it’s someone else”)

Why is einschlafen at the end, and what kind of verb is it?

einschlafen is a separable verb:

Prefix: ein-
Main part: schlafen (to sleep)
Meaning together: einschlafen = “to fall asleep”

With a modal verb (here: kann), the structure is:

Conjugated modal verb (in 2nd position)
Subject
Other stuff (objects, adverbs, nicht, etc.)
Infinitive (here: einschlafen) at the end of the clause

So in:

Manchmal kann ich nicht einschlafen.

you have:

kann (modal, conjugated)
ich (subject)
nicht (negation)
einschlafen (infinitive of the main verb at the end)

If there were no modal verb, the separable verb would split in a main clause:

Ich schlafe ein. (present, no modal)

But with a modal:

Ich kann nicht einschlafen.
- not: ❌ Ich kann nicht schlafe ein.

Why is it den nächsten Tag and not dem nächsten Tag?

The case is determined by the verb–preposition combination an … denken.

denken an + accusative = to think of / about (something)

Because an with denken takes the accusative, Tag must be in the accusative case:

Masculine noun der Tag:
- nominative: der Tag
- accusative: den Tag
- dative: dem Tag

Here we need the accusative, so:

an den nächsten Tag (not an dem nächsten Tag)

Compare:

Ich denke an den Urlaub. (accusative, intention: I think about the vacation.)
Ich erinnere mich an den Urlaub. (also accusative)

Different verbs + an can use different cases, e.g.:

am Tag = an dem Tag (here an is used with dative without denken; it means “on that day” in a temporal sense)

What is the difference between an den nächsten Tag denken and über den nächsten Tag nachdenken?

Both can be translated as “to think about the next day”, but they have slightly different nuances:

an den nächsten Tag denken
- Verb: denken an + accusative
- Meaning: simply to think of / about something; it can be brief or not very deep.
- In context: your mind goes to the next day; you picture it, worry about it, anticipate it, etc.
über den nächsten Tag nachdenken
- Verb: nachdenken über + accusative
- Meaning: to reflect on, to think something through, more deliberate, often somewhat deeper thinking.
- In context: you are really considering, planning, analyzing the next day.

So:

Ich kann nicht einschlafen, weil ich an den nächsten Tag denke.
- Sounds like: My thoughts keep going to the next day (worries, plans, expectations), so I can’t sleep.
… weil ich über den nächsten Tag nachdenke.
- Emphasizes: I am actively, consciously thinking/reflecting about the next day (maybe making decisions or planning), which keeps me awake.

Why is nicht placed before einschlafen and not somewhere else?

In German, nicht usually comes before:

the part of the predicate it negates (especially before an infinitive at the end),
or before an adverb / prepositional phrase / object that is being negated.

In kann ich nicht einschlafen:

The part being negated is the ability to fall asleep (the infinitive einschlafen).
Standard word order with a modal verb puts nicht directly before the infinitive:
- Ich kann nicht einschlafen.
- Ich will nicht arbeiten.
- Ich darf heute nicht fernsehen.

If you moved nicht somewhere else, it would either sound wrong, or put focus/contrast on a different part of the sentence. For a neutral statement, nicht goes right before einschlafen.

Why is the verb at the end in weil ich an den nächsten Tag denke?

weil introduces a subordinate clause (a dependent clause that gives a reason: “because …”).

In German subordinate clauses, the conjugated verb normally goes to the very end of the clause.

Structure of the subordinate clause here:

Conjunction: weil
Subject: ich
Object / prepositional phrase: an den nächsten Tag
Conjugated verb: denke (moved to the end)

So:

…, weil ich an den nächsten Tag denke.

Compare with a main clause version:

Main clause: Ich denke an den nächsten Tag. (verb is 2nd)
Sub clause: …, weil ich an den nächsten Tag denke. (verb at the end)

This is a standard rule: weil, dass, wenn, obwohl, als, etc. all send the conjugated verb to the end of their clause.

Could I say …, weil ich an morgen denke instead of an den nächsten Tag?

Yes, …, weil ich an morgen denke. is grammatically correct and natural.

Nuance:

an den nächsten Tag
- Slightly more formal or neutral.
- Very concrete: “the next day” (e.g., tomorrow in relation to tonight, or the next day in a narrative).
an morgen
- Literally “about tomorrow”.
- More colloquial, more commonly used in everyday speech when you mean “tomorrow” from now.

In the context of going to sleep today, both can mean “because I’m thinking about tomorrow”. Often you’d hear:

Manchmal kann ich nicht einschlafen, weil ich an morgen denke.

Why is the tense present (kann, denke) if the problem is about the future (the next day)?

German, like English, often uses the present tense to talk about future events, especially when:

the future is near,
the future is scheduled or expected,
or the time reference is made clear by words like morgen, nächste Woche, bald, später, am Montag, etc.

In this sentence:

kann ich nicht einschlafen = present tense, describing a regular / habitual problem.
ich an den nächsten Tag denke = present tense, but “der nächste Tag” itself is a future time.

The time reference (der nächste Tag) already makes clear we’re dealing with future events (tomorrow or the following day). No Futur I (werde) is needed here.

You’d use Futur I more for emphasis, predictions, or when there’s no clear time word, for example:

Morgen werde ich viel arbeiten.
(But Morgen arbeite ich viel is also very common and natural.)

Is there any difference between starting with Manchmal and starting with Ich?

Yes, it slightly changes the focus and the rhythm, but not the core meaning.

Manchmal kann ich nicht einschlafen, weil ich an den nächsten Tag denke.
- Starts with a time adverb: Manchmal (sometimes).
- Puts emphasis on the “sometimes”: it doesn’t happen all the time.
Ich kann manchmal nicht einschlafen, weil ich an den nächsten Tag denke.
- Starts with Ich (I).
- Emphasis a bit more on “I” as the topic, with manchmal modifying kann nicht einschlafen in the middle.

Both are correct. The first version (with Manchmal first) sounds slightly more natural when you specifically want to highlight irregularity:

It’s not every night, just sometimes, this happens.

Why is there a comma before weil? Could I leave it out?

In German, subordinate clauses are normally set off by commas.

weil introduces a subordinate clause, so a comma is required before it:
- Manchmal kann ich nicht einschlafen, weil ich an den nächsten Tag denke.

You cannot leave the comma out in standard written German. It is not optional in this case.

The comma helps the reader recognize where the main clause ends and the subordinate clause begins:

Main clause: Manchmal kann ich nicht einschlafen
Subordinate clause: weil ich an den nächsten Tag denke

Could I use denn instead of weil, and what would change?

You could say:

Manchmal kann ich nicht einschlafen, denn ich denke an den nächsten Tag.

Differences between weil and denn:

Grammar / word order
- weil introduces a subordinate clause → verb at the end:
  - …, weil ich an den nächsten Tag denke.
- denn introduces a main clause → verb in 2nd position:
  - …, denn ich denke an den nächsten Tag.
Style / feeling
- weil is more neutral and very common in speech and writing.
- denn sounds a bit more written, explanatory, sometimes slightly more formal or old-fashioned in everyday speech.

Meaning-wise, both give a reason (“because”), but weil is more common in spoken German in this kind of sentence.