As a general rule, the preposition "in" (+ Accusative) is used for going to places which one will then be "inside". To describe being "inside" such places - which include rooms and most buildings - you should use "in" (+ Dative). When leaving such places, you use the preposition "aus". This preposition always takes the dative case, even though motion is involved.
Grammar 27: "in" und "aus"
Ich gehe ins Badezimmer.
("I go into the bathroom.")
Ich gehe in die Küche.
("I go into the kitchen.")
Ich bin im Badezimmer.
("I am in the bathroom.")
Ich bin in der Küche.
("I am in the kitchen.")
Ich gehe aus dem Badezimmer.
("I go out of the bathroom.")
Ich gehe aus der Küche.
("I go out of the kitchen.")
Notes
Point 1German also uses the preposition "in" (+ Accusative) wher we would not necessarily do so in English:
Gehen wir ins Konzert!
(Let's go to the concert!)
Herr Dr. Schmidt geht regelmäßig ins Theater.
(Dr. Schmidt regularly goes to the theatre.)
Ich gehe ins Stadtzentrum!
(I'm going to the city centre.)
Im Winter fahren wir in die Berge.
(In winter we go into the mountains.)
Point 2For a number of German buildings and public places, you should use the preposition "zu" (+ Dative) for motion towards, "an" (+ Dative) to translate position ("at" or "in") and "von" (+ Dative) to translate motion away. We have already dealt with this grammatical point in Chapter 6, wher the correct prepositions for travelling to and from countries are also explained.
Point 3When using the separable verb ankommen (= "to arrive"), the place wher you arrive is in the dative case:
Ich komme um Viertel vor acht im Büro an.
(I arrive in the office at a quarter to eight.)
Wann kommt sie am Flughafen an?
(When does she arrive at the airport?)
Point 4Note too that the same prepositions are used for travelling to and from towns and cities as are used for travelling to (neuter) countries.
Grammar 28: Prepositions and cities
Wir fahren nach Werder.
("We travel to Werder.")
Mareike wohnt in Berlin.
("Mareike lives in Berlin.")
Peter kommt aus Nürnberg.
("Peter comes from Nuremberg.")