I’m having a really busy day, but thankfully, I’ve managed to squeeze in a quick bit of revision. This is about using the prepositions in and an.
The following sentences indicate motion, or more specifically, motion towards something.
Ich gehe ins Museum – I am going to the museum.
Wir woollen an einen See fahren – We want to go to a lake.
Meine Mutter geht gern in die Oper – My mother likes going to the opera.
The prepositions in and an take the accusative case when used with a verb of motion (gehen, fahren, etc). You use an if you are going towards water (ans Meer, an den See, an den Fluss), otherwise use in (ins Theater, in den Park).
It’s worth noting that ins is normally used instead of in das, and ans is used instead of an das. (Something to find out about: would it still be correct to use in das and an das?)
When the verb does not indicate motion, the prepositions take the dative case. That, however, is something for another day!