Meine Schwester sucht ihren Gürtel, bevor sie das Haus verlässt.

Because suchen takes a direct object, and direct objects are in the accusative case.

• der Gürtel = the belt (masculine nominative)
• accusative masculine changes the article/pronoun form

So:

• ihr Gürtel = her belt as a subject
• ihren Gürtel = her belt as a direct object

In this sentence, her belt is the thing being looked for, so German uses ihren Gürtel.

The possessor is meine Schwester, so the belt belongs to her. German uses the possessive stem ihr- for her.

Here it appears as ihren because it has to match:

• the owner: her
• the noun: Gürtel (masculine singular)
• the case: accusative

So ihren Gürtel means her belt.

A useful note: ihr-/ihren can also mean other things in other contexts, such as their or formal your, but here the context clearly makes it her.

Because it is the subject of the sentence, so it is in the nominative case.

• die Schwester = the sister
• meine Schwester = my sister

For a feminine noun in the nominative singular, mein- becomes meine.

So:

• meine Schwester = subject form
• meiner Schwester would be a different case, such as dative or genitive in some contexts

Because bevor sie das Haus verlässt is a new clause, and German normally states the subject of that clause explicitly.

So the sentence has two clauses:

1. Meine Schwester sucht ihren Gürtel
2. bevor sie das Haus verlässt

In the second clause, sie refers back to meine Schwester.

Because bevor is a subordinating conjunction. In German, a subordinating conjunction sends the finite verb to the end of its clause.

So:

• sie verlässt das Haus = normal main clause order
• bevor sie das Haus verlässt = subordinate clause, verb moves to the end

This is one of the most important German word-order patterns to learn.

Bevor means before, but specifically when it introduces a clause.

So:

• bevor sie das Haus verlässt = before she leaves the house

Compare that with vor, which is usually a preposition, not a conjunction.

For example:

• vor dem Haus = in front of the house / before the house

So bevor is used with a full clause, while vor is used with a noun phrase.

Because verlassen is a transitive verb in German. It takes a direct object.

So German says:

• das Haus verlassen = literally leave the house

English often works similarly here, but learners sometimes expect a preposition because German often uses them in other expressions.

You can say:

• Sie verlässt das Haus. = She leaves the house.

Not:

• Sie verlässt aus dem Haus. ✘

No. Verlassen is not separable.

That is why the form is:

• sie verlässt das Haus

and not something like:

• sie lässt das Haus ver ✘

In the perfect tense, you can also see that it behaves like a normal inseparable verb:

• Sie hat das Haus verlassen.

Because Haus is a neuter noun, and in German the article das is the same in both the nominative and the accusative singular.

So:

• nominative: das Haus
• accusative: das Haus

Even though the form does not change, the function does: here it is the direct object of verlässt.

This can be confusing because with masculine nouns, the accusative usually looks different, but with neuter nouns it often does not.

Yes. German allows that.

You could also say:

• Bevor sie das Haus verlässt, sucht meine Schwester ihren Gürtel.

That means the same thing.

When the subordinate clause comes first, the main-clause verb still has to be in the second position, so you get:

• Bevor sie das Haus verlässt, sucht ...

not:

• Bevor sie das Haus verlässt, meine Schwester sucht ... ✘

This is a very common German word-order rule.

It is:

• Main clause: Meine Schwester sucht ihren Gürtel
• Subordinate clause: bevor sie das Haus verlässt

So the overall pattern is:

[main clause], bevor [subject ... verb-at-the-end]

That gives you a useful model for many similar sentences, such as:

• Ich esse, bevor ich arbeite.
• Er ruft an, bevor er kommt.