Breakdown of Die Katze läuft hinter den Vorhang.
die Katze
the cat
hinter
behind
laufen
to walk / to run (steady pace, jog)
der Vorhang
the curtain
Questions & Answers about Die Katze läuft hinter den Vorhang.
Why is it den Vorhang instead of dem Vorhang?
hinter is a two-way preposition. Because the cat is moving towards a new destination (answering the question of direction, wohin?), we use the accusative case. We only use the dative case for these prepositions when there is no change in location.
Would it make sense to say Die Katze läuft hinter dem Vorhang?
Yes, but it completely changes the meaning! If you use the dative case (dem Vorhang), it means the action is happening entirely in one fixed location. It would mean the cat is already located behind the curtain and is running around back there.
Why does the verb laufen have an umlaut here (läuft)?
The verb laufen (to run or walk) is a strong verb that changes its stem vowel from a to ä for the du and er/sie/es forms. Since die Katze acts as a third-person subject, the verb becomes läuft.
What gender is Vorhang, and why does it matter here?
It is masculine (der Vorhang). This is important because masculine nouns are the only ones that change their definite article in the accusative case (from der to den). If the curtain were a feminine or neuter noun, the accusative article would look exactly the same as it does in the nominative case.