Learn Hardcore German: In diesem Albtraum hatte ich meine Hausaufgaben vergessen und stand allein im Klassenzimmer. - In this nightmare I had forgotten my homework and was standing alone in the classroom.

German main clauses follow the verb‑second (V2) rule: the conjugated verb must be in second position, but anything can stand in first position (subject, object, time expression, etc.).

Here, In diesem Albtraum (a prepositional phrase of “situation/time”) is put first for emphasis or style.
The finite verb hatte must then come second, so the subject ich moves after the verb:
Neutral order: Ich hatte in diesem Albtraum meine Hausaufgaben vergessen.
Fronted phrase: In diesem Albtraum hatte ich meine Hausaufgaben vergessen.

Both are correct; the second just emphasizes the nightmare setting more.

Why is it hatte ich and not ich hatte after In diesem Albtraum?

Because of the V2 rule:

Slot 1: In diesem Albtraum (first constituent)
Slot 2: hatte (finite verb must be second)
Slot 3+: ich meine Hausaufgaben vergessen

You can only have one element before the conjugated verb in a main clause. So you cannot say:

✗ In diesem Albtraum ich hatte meine Hausaufgaben vergessen.

That would violate verb‑second word order. Hence: In diesem Albtraum hatte ich ….

Why is hatte (past perfect) used instead of habe or a simple past like vergaß?

The choice of tense signals relative time inside the story:

hatte vergessen = past perfect (“had forgotten”)
- This describes something that was already completed before another past situation.
stand = simple past (“stood” / “was standing”)

So in the story:

First, in the nightmare, I forgot my homework (completed earlier → hatte vergessen).
After that, I stood alone in the classroom (later past situation → stand).

Alternatives:

In diesem Albtraum habe ich meine Hausaufgaben vergessen …
→ Perfect; more typical for spoken German, less clear about sequence in written narrative.
In diesem Albtraum vergaß ich meine Hausaufgaben …
→ Simple past; grammatically fine, sounds more literary.

The past perfect hatte vergessen stresses that the forgetting was prior to the standing.

Why is vergessen at the end of the first part of the sentence?

In compound tenses (like the perfect and past perfect), German sends the past participle to the end of the clause:

Auxiliary verb (conjugated) in 2nd position: hatte
Past participle at the end: vergessen

Pattern:

Ich hatte meine Hausaufgaben vergessen.
Ich habe das Buch gelesen.
In diesem Albtraum hatte ich meine Hausaufgaben vergessen.

So hatte … vergessen is split, with vergessen in final position because of German word order rules.

Why is it diesem Albtraum and not dieser Albtraum or diesen Albtraum?

This is a case + gender issue.

Albtraum has gender masculine: der Albtraum.
The preposition in with a static situation/time (“in this nightmare”, not “into this nightmare”) takes the dative.
Masculine dative singular of dieser is diesem.

Declension of dieser (masculine, singular):

Nominative: dieser Albtraum (this nightmare – as subject)
Accusative: diesen Albtraum (I have this nightmare)
Dative: diesem Albtraum (in/with/from this nightmare)
Genitive: dieses Albtraums

Since in diesem Albtraum describes the situation (“in this nightmare” = location/time), we use dative → diesem.

Why is it meine Hausaufgaben and not meiner Hausaufgaben?

Again, this is case:

meine Hausaufgaben is the direct object of vergessen.
Direct objects in German usually take the accusative case.

The pronoun mein‑ in the plural accusative is meine:

Nominative plural: meine Hausaufgaben
Accusative plural: meine Hausaufgaben (same form)
Dative plural: meinen Hausaufgaben
Genitive plural: meiner Hausaufgaben

Since they are the thing that was forgotten (accusative object), we use meine, not meiner.

Why is Hausaufgaben plural in German, when English usually says homework (singular)?

The two languages conceptualize this differently:

English homework is usually an uncountable noun.
German die Hausaufgabe is countable: literally “house-task,” a single assignment.

Common patterns:

die Hausaufgabe – one homework task
die Hausaufgaben – several tasks / “homework” as a whole

In practice, Hausaufgaben (plural) is very frequent for what English speakers just call homework:

Ich habe viele Hausaufgaben. – “I have a lot of homework.”
Ich habe meine Hausaufgaben vergessen. – “I forgot my homework.”

Why is there no article before Hausaufgaben? Why not die Hausaufgaben?

There is a determiner: meine.

German noun phrases typically have one article-like element:

either a definite/indefinite article (der, ein, etc.)
or a possessive/other determiner (mein, dieser, etc.)
or both in some special structures, but normally just one.

So you say:

meine Hausaufgaben (my homework)
die Hausaufgaben (the homework)
but not ✗ die meine Hausaufgaben in this simple context.

Here, meine already specifies the noun, so no extra article is used.

Why is stand in the simple past, while hatte vergessen is in the past perfect? Isn’t mixing tenses wrong?

Mixing these two tenses is not wrong; it’s actually typical in narratives:

hatte vergessen (past perfect) = an action completed before another past action.
stand (simple past) = the main narrative past.

Timeline in the nightmare:

First: I had forgotten my homework → hatte … vergessen
Then: I stood alone in the classroom → stand

German frequently uses past perfect for “earlier past” and simple past for the “current” narrative scene. So combining hatte vergessen and stand is good style.

Could I say … hatte ich meine Hausaufgaben vergessen und hatte allein im Klassenzimmer gestanden instead?

You could, but it sounds unnatural and changes the nuance:

hatte … vergessen und stand …
→ First the forgetting (earlier past), then the “standing” is the main scene.
hatte … vergessen und hatte allein im Klassenzimmer gestanden
→ Both actions are pushed into “earlier past” relative to some other (later) reference point, and you also repeat the auxiliary, which adds heaviness.

In most storytelling contexts, the original is better:

Past perfect for the background event (hatte vergessen)
Simple past for the main scene (stand)

Why is it im Klassenzimmer and not in dem Klassenzimmer?

im is just the contracted form of in dem:

in + dem = im

We use dem because:

Klassenzimmer is neuter: das Klassenzimmer
in with a location (“in the classroom” = where?) takes dative
Neuter dative singular of der/das/die is dem

So:

Full: in dem Klassenzimmer
Normal contracted form: im Klassenzimmer

Both are grammatically correct; im is far more common in everyday language.

How do we know that Klassenzimmer is neuter (das Klassenzimmer)?

Klassenzimmer is a compound noun: Klasse + Zimmer.

For German compounds, the last element decides the gender:

die Klasse (class) – feminine
das Zimmer (room) – neuter

Since Zimmer is neuter, the whole compound Klassenzimmer is neuter:

das Klassenzimmer
Dative singular: dem Klassenzimmer → im Klassenzimmer

Why is Albtraum capitalized?

In German, all nouns are capitalized, regardless of their position in the sentence.

Albtraum is a noun (meaning nightmare), so it must be written with a capital A.
The same applies to Hausaufgaben and Klassenzimmer.

This is a fixed rule of German spelling and is one of the main visual differences from English.

Why is there no comma before und stand?

Because hatte … vergessen and stand share the same subject ich and form a joint predicate in one main clause:

Subject: ich
Verbs: hatte … vergessen und stand …

In such cases, German does not use a comma:

In diesem Albtraum hatte ich meine Hausaufgaben vergessen und stand allein im Klassenzimmer.

If you repeated the subject, you’d usually treat them as two main clauses and use a comma:

In diesem Albtraum hatte ich meine Hausaufgaben vergessen, und ich stand allein im Klassenzimmer.

Both versions are possible, but the original (without comma, shared subject) is more natural here.