Learn Hardcore German: Auf der Klassenfahrt schlafen wir in einer Jugendherberge im großen Schlafsaal. - On the class trip we sleep in a youth hostel in the big dormitory.

In the sentence, the meaning is “during the class trip / on the class trip” → this is a situation/location in time, not movement towards it.
So you use dative:

auf der Klassenfahrt = on/during the class trip (where we are)
auf die Klassenfahrt fahren = to go on the class trip (movement towards it → accusative)

But “Klassenfahrt” is feminine (die Klassenfahrt). Why do we see “der Klassenfahrt”?

You’re seeing the dative singular of a feminine noun.

Nominative: die Klassenfahrt
Accusative: die Klassenfahrt
Dative: der Klassenfahrt
Genitive: der Klassenfahrt

Because auf here requires dative (location), die Klassenfahrt changes to der Klassenfahrt:

auf der Klassenfahrt = on/during the class trip

Why does the sentence start with “Auf der Klassenfahrt” and then the verb comes right after: “schlafen wir …”?

German main clauses follow the verb-second rule (V2):

The conjugated verb is always in second position in a main clause.
“Second position” means: second element, not second word.

Here, the first element is a prepositional phrase:

Auf der Klassenfahrt (element 1)
schlafen (conjugated verb – element 2)
wir (subject – element 3)
Rest of the sentence…

You could also say:

Wir schlafen auf der Klassenfahrt in einer Jugendherberge im großen Schlafsaal.

Now wir is element 1, and schlafen is still in second position. Both word orders are correct; putting “Auf der Klassenfahrt” first just emphasizes the trip.

Why is the present tense “schlafen wir” used when this is about the future?

German very often uses the present tense to talk about the near future, especially for planned events:

Morgen fahren wir nach Berlin. = Tomorrow we’re going to Berlin.
Nächste Woche schreiben wir eine Klassenarbeit. = Next week we’ll write a test.

So:

Auf der Klassenfahrt schlafen wir …
= On the class trip we will sleep …

Using the future tense (werden schlafen) is possible but less common here and sounds more formal or more strongly predictive:

Auf der Klassenfahrt werden wir in einer Jugendherberge schlafen.

Why is it “in einer Jugendherberge” and not “in eine Jugendherberge”?

The preposition in can also take dative (location) or accusative (direction):

Dative = where something is / where something happens
Accusative = where something moves to

Here, the meaning is “we will be staying/sleeping in a youth hostel” → location, not movement, so you use dative:

Feminine noun: die Jugendherberge
Dative singular feminine: einer Jugendherberge

So:

Wir schlafen in einer Jugendherberge. (location → dative)
Wir gehen in eine Jugendherberge. (movement into → accusative)

What exactly is a “Jugendherberge”? Is it just “youth hostel”?

Jugendherberge is usually translated as youth hostel, but it often carries a bit more specific nuance:

Traditionally, Jugendherbergen in German-speaking countries are cheap, simple accommodations, often part of an official organization like the Deutsches Jugendherbergswerk (DJH).
They are very common for school trips, youth groups, and clubs.
They usually have shared dorm rooms, sometimes rules (e.g. no alcohol), and are more structured and group-oriented than many independent hostels.

So “Jugendherberge” overlaps with “youth hostel”, but especially suggests a school/youth group setting, which fits the idea of a class trip (Klassenfahrt).

What does “im” in “im großen Schlafsaal” mean exactly?

“im” is a contraction of:

in + dem → im

So:

im großen Schlafsaal literally = in dem großen Schlafsaal = in the big dormitory.

Here, dem is dative singular masculine (or neuter), required by in when it expresses location.

Why is it “im großen Schlafsaal” and not “im groß Schlafsaal” or “im großer Schlafsaal”?

Because of adjective endings in German.
Let’s break it down:

Schlafsaal = masculine noun, singular
Article: dem (inside im = in dem) → definite article, dative masculine singular
After a definite article (dem), adjectives get the weak ending.

Dative masculine singular, weak ending → -en:

in dem großen Schlafsaal → im großen Schlafsaal

So:

groß (dictionary form)
großen (correct form here with article dem in dative masculine)

“im groß Schlafsaal” or “im großer Schlafsaal” are grammatically incorrect.

What case is “im großen Schlafsaal”, and why?

“im großen Schlafsaal” is in the dative case.

Reason:

The preposition in takes dative when expressing location (where something is), and accusative for direction (where something goes).
In this sentence, it’s a location: we sleep in the big dormitory.

So you need dative:

Masculine noun: der Schlafsaal
Dative singular masculine: dem Schlafsaal
Contraction: in dem → im
Adjective with definite article in dative masculine: großen

Result: im großen Schlafsaal.

What is a “Schlafsaal”, and why is it masculine?

Schlafsaal is a compound noun:

schlafen = to sleep
der Saal = hall, large room
Schlafsaal = dormitory-style sleeping hall, big shared bedroom with many beds

The gender of a compound noun in German is determined by the last part (the head).

Last part here: der Saal (masculine)
Therefore: der Schlafsaal is masculine.

That’s why in dative singular you get dem Schlafsaal, which becomes im Schlafsaal.

Is there a rule for the order of all these phrases: “Auf der Klassenfahrt schlafen wir in einer Jugendherberge im großen Schlafsaal.”?

Two ideas help explain the order:

Verb-second rule
Something must come first, then the verb in second position. Here, the speaker puts the time/situation phrase first:
- Auf der Klassenfahrt (when/under what circumstance)
- schlafen wir (verb + subject)
- in einer Jugendherberge im großen Schlafsaal (where)
From general to specific location
Within the location, German (and English) often go from general → specific:
- in einer Jugendherberge (general location)
- im großen Schlafsaal (more specific: which part of the Jugendherberge)

You could also say:

Auf der Klassenfahrt schlafen wir im großen Schlafsaal in einer Jugendherberge.

This is still correct but sounds a bit less natural; general → specific (Jugendherberge → Schlafsaal) is nicer.

Could you say “Auf die Klassenfahrt schlafen wir …”? What would that mean?

No, “Auf die Klassenfahrt schlafen wir …” is not correct German in this context and basically doesn’t make sense.

auf die Klassenfahrt (accusative) would suggest movement towards the trip, which isn’t logical with schlafen (to sleep).
You’d use accusative with a verb of movement:
- Wir freuen uns auf die Klassenfahrt. = We are looking forward to the class trip.
- Wir fahren auf die Klassenfahrt. = We are going on the class trip.

With schlafen (a position/activity verb), you want to describe where you are when you sleep, so you need dative:

Auf der Klassenfahrt schlafen wir …

Could you also use “übernachten” instead of “schlafen” here?

Yes. übernachten = to stay overnight / to spend the night and is often more idiomatic for talking about accommodation:

Auf der Klassenfahrt übernachten wir in einer Jugendherberge im großen Schlafsaal.

Subtle difference:

schlafen = literally sleeping (the act of sleeping)
übernachten = where you stay the night, focusing more on the accommodation aspect

Both are correct; übernachten might sound a bit more natural if you specifically talk about where you’re lodging.

Why are “Klassenfahrt, Jugendherberge, Schlafsaal” capitalized?

In German, all nouns are capitalized, regardless of where they appear in the sentence.

die Klassenfahrt → Klassenfahrt
die Jugendherberge → Jugendherberge
der Schlafsaal → Schlafsaal

This includes compound nouns like Klassenfahrt (Klasse + Fahrt) and Schlafsaal (Schlaf + Saal).