Questions & Answers about Der Hund geht hinter das Sofa.
Why is it das Sofa and not dem Sofa here?
Because hinter (behind) is a two-way preposition, and the dog is moving to a new destination (going behind the sofa). When a two-way preposition answers the question Where to? (Wohin?), it takes the accusative case. Since Sofa is a neuter noun, the accusative word for the is das.
How would I say 'The dog is walking around behind the sofa'?
If the dog is already behind the sofa and just walking around back there, there is no change of destination. It answers Where? (Wo?) instead of Where to?. In that case, you use the dative case: Der Hund geht hinter dem Sofa.
Can geht mean both 'goes' and 'is going'?
Yes! German does not have a separate continuous tense like English does with -ing. Der Hund geht translates to both 'The dog goes' and 'The dog is going', depending on the context.
Can I combine hinter and das into a shorter word like ins?
No. While you can combine in and das to make ins, or an and das to make ans, you cannot contract hinter das. You must write and say both words separately.