You usually add -r or -er to the ending, like -s in English.
But because Danish changes noun endings instead of using "the" in front of them, it's different for definite nouns. Plural definite nouns usually get the ending -rne/-erne/-ene (depending on whether they're common gender or neuter, in definite singular, common gender nouns tend to end in -n, and neuter end in -t).
Examples, English - Danish
(common)
a staircase - en trappe
the staircase - trappen
two staircases - to trapper
all the staircases - alle trapperne
(neuter)
a sign - et skilt
the sign - skiltet
two signs - flere skilte
all the signss - alle skiltene
(have fun)
four fire alarms made Buster fall into the trap at the end of the staircases at high speed
- fire brandalarmer fik Buster til at falde i fælden for enden af trappen med høj fart
