Learn Hardcore German: Bevor wir schlafen, legt meine Schwester ihren Schlafsack ins Zelt und füllt die Thermosflasche mit heißem Wasser. - Before we sleep, my sister puts her sleeping bag into the tent and fills the thermos flask with hot water.

Main clauses: the finite verb is in second position.
Subordinate clauses (introduced by words like bevor, weil, dass, wenn, obwohl): the finite verb goes to the end.

So:

Main clause: Wir schlafen früh. – We sleep early.
Subordinate clause: Bevor wir schlafen, … – Before we sleep, …

That’s why it’s Bevor wir schlafen and not Bevor wir schlafen wir or Bevor wir wir schlafen. The verb must be last in the subordinate clause.

Why is it legt meine Schwester … and not meine Schwester legt … after the comma?

In the main clause legt meine Schwester ihren Schlafsack ins Zelt und füllt …, the verb legt comes first because something else has already been put in “first position.”

The structure is:

Bevor wir schlafen, → this whole subordinate clause occupies the first position of the main clause.
Then the finite verb must be in second position, so legt comes next.
The subject meine Schwester follows after the verb.

So the pattern is:

[1: Bevor wir schlafen], [2: legt] [3+: meine Schwester … ]

If you don’t start with a subordinate clause, the subject can be in first position:

Meine Schwester legt ihren Schlafsack ins Zelt.

Both are correct German; the position of legt is determined by the rule “verb in second position” in the main clause.

Why is there a comma after schlafen?

Because German always uses a comma to separate a subordinate clause from a main clause.

Here:

Subordinate clause: Bevor wir schlafen
Main clause: legt meine Schwester ihren Schlafsack ins Zelt und füllt die Thermosflasche mit heißem Wasser.

If you swap the order, the comma is still there, just in a different place:

Meine Schwester legt ihren Schlafsack ins Zelt und füllt die Thermosflasche mit heißem Wasser, bevor wir schlafen.

In both orders, the comma marks the boundary between the two clauses.

Why is it ihren Schlafsack instead of ihr Schlafsack, and who does ihren refer to?

ihren is the accusative singular masculine form of the possessive pronoun ihr- (her).

Schlafsack is masculine: der Schlafsack.
In this sentence, Schlafsack is the direct object (what is being laid), so it is in the accusative case.
Masculine accusative with ihr- takes the ending -en → ihren Schlafsack.

Compare:

Nominative (subject): Ihr Schlafsack ist neu. – Her sleeping bag is new.
Accusative (object): Sie legt ihren Schlafsack ins Zelt. – She puts her sleeping bag in the tent.

ihren refers back to meine Schwester – it means her (my sister’s) sleeping bag.
In other contexts, ihren can also mean their or (capitalized) Your (formal), but here the immediate antecedent meine Schwester makes her the intended meaning.

Why is it ins Zelt and not im Zelt?

Because the sentence describes movement into the tent, not a position inside the tent.

With in, German distinguishes:

in + accusative → movement/direction (into something)
in + dative → location (in/inside something, no movement)

Here:

legt … ins Zelt = lays it into the tent → movement → accusative
- ins = in das (in + the, accusative)
liegt im Zelt = lies in the tent (no movement) → dative
- im = in dem (in + the, dative)

So ins Zelt is correct because she is putting the sleeping bag into the tent.

What exactly is ins here?

ins is a contraction of in das.

Preposition: in
Definite article, neuter accusative: das (because das Zelt)
Spoken and written German often merges them: in das Zelt → ins Zelt.

Similarly:

in dem Haus → im Haus
zu dem Arzt → zum Arzt
an das Meer → ans Meer

So ins Zelt literally means into the tent.

Why is Thermosflasche feminine (die Thermosflasche)?

In German compound nouns, the last part of the compound determines the grammatical gender.

Thermosflasche = Thermos + Flasche

die Flasche (bottle) is feminine.
Therefore, die Thermosflasche is also feminine.

So you say:

die Thermosflasche (nominative)
ich habe eine Thermosflasche (accusative)
mit der Thermosflasche (dative, with the Thermos bottle)

Why is it mit heißem Wasser and not mit heißes Wasser or mit heißen Wasser?

The form heißem is required because:

mit always takes the dative case.
Wasser is neuter: das Wasser.
Neuter dative singular with no article uses the adjective ending -em: heißem Wasser.

Very schematically:

Neuter nominative/accusative (no article): heißes Wasser
Neuter dative (no article): heißem Wasser

So:

Ich trinke heißes Wasser. – I drink hot water. (accusative)
Sie füllt die Thermosflasche mit heißem Wasser. – … with hot water. (dative after mit)

That’s why mit heißem Wasser is the correct form here.

Why is there no article before heißem Wasser?

Because Wasser is treated as an uncountable / mass noun here, and German often omits the article when speaking about an indefinite, nonspecific amount.

You can think of it as:

mit heißem Wasser → with (some) hot water (general, unspecific)
mit dem heißen Wasser → with the hot water (a specific, already known water, e.g. “the hot water we boiled earlier”)

Both are grammatically correct; mit heißem Wasser is simply the more natural choice when the water is not yet specifically identified in the discourse.

What is the difference between legen and liegen, and why is legt used here?

legen and liegen are a typical German verb pair:

legen – to lay / put something down (transitive, you need an object)
- Sie legt den Schlafsack ins Zelt. – She lays the sleeping bag in the tent.
liegen – to lie (be in a lying position), no object
- Der Schlafsack liegt im Zelt. – The sleeping bag is lying in the tent.

In the sentence, we talk about the action of putting the sleeping bag into the tent, so the verb must be legen → legt (3rd person singular).

If you describe the result afterwards, you would use liegen:

Jetzt liegt der Schlafsack im Zelt. – Now the sleeping bag is in the tent (lying there).

Why is the present tense (wir schlafen) used instead of a future form?

German normally uses the present tense to talk about the near future, especially when there is a time-related word or clause like morgen, später, wenn, bevor, etc.

So:

Bevor wir schlafen, … literally: Before we sleep, …
but in context it clearly means before we go to sleep (later).

A future form like Bevor wir schlafen werden is grammatically possible but sounds unnatural or overly formal here. Native speakers prefer the simple present.

Could the word order also be Meine Schwester legt ihren Schlafsack ins Zelt und füllt die Thermosflasche mit heißem Wasser, bevor wir schlafen? Does the meaning change?

Yes, that word order is completely correct, and the meaning does not change.

You have two options:

Subordinate clause first:
- Bevor wir schlafen, legt meine Schwester …
Main clause first:
- Meine Schwester legt …, bevor wir schlafen.

In both versions, bevor wir schlafen expresses the same time relationship: her actions happen before sleeping. Changing the order just changes the focus / rhythm of the sentence, not the basic meaning.

Why are Schlafsack, Zelt, Thermosflasche, and Wasser capitalized?

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

der Schlafsack – sleeping bag
das Zelt – tent
die Thermosflasche – Thermos bottle
das Wasser – water

Adjectives like heißem and verbs like legt, füllt, schlafen are not capitalized (except at the start of a sentence).