Use -en after the dative determiners taught here
The adjective ending is the same in dem alten Bahnhof, der alten Schule, einem alten Gebäude and meinen alten Büchern. The determiner still needs the right gender, number and case.
Wir treffen uns vor der alten Schule.
We are meeting in front of the old school.
A contracted article still counts
Im combines in dem; am combines an dem. The article is still part of the phrase, so use forms such as im alten Gebäude and am alten Bahnhof.
Sie wartet am alten Bahnhof.
She is waiting at the old station.
The plural noun may need another ending
In mit den alten Büchern, the adjective ends in -en, while the noun changes from Bücher to Büchern. Plurals already ending in -n or -en, and plurals in -s, do not add another dative -n.
Ich arbeite mit den alten Computern.
I work with the old computers.
Location can include movement
Wir tanzen im großen Zimmer describes dancing within the room: dative. Wir gehen in das große Zimmer describes entering it: accusative. With two-way in, the contrast is location versus destination, not stillness versus movement.
Wir tanzen im großen Zimmer.
We are dancing in the large room.
Without a determiner, do not blindly use -en
The rule needs its scope. Compare mit dem kalten Wasser and mit kaltem Wasser: no-article dative neuter takes -em. Learn the determiner pattern and the no-article pattern separately.
Ich wasche das Glas mit kaltem Wasser.
I wash the glass with cold water.
Practise this pattern
This explanation is free to read. The matching A2 block includes guided practice, a separate block check and spaced review. The first 10 blocks of each language stay free; other interactive lessons require course access or an eligible seven-day trial.
Explore this grammar blockRelated grammar
References
Original teaching prose and examples, informed by these grammar references. A2 placement is an editorial curriculum choice, not a CEFR certification.