When an adjective appears after a noun, it does not have an ending:
Das Haus ist alt ... the house is old.
When adjectives appear before a noun, however, it is much more troublesome! Look at the following table (click here to revise cases):
A bit daunting, isn't it?! Now look again...
There are actually only two possible adjective endings after "the" (der/die/das etc): -e and -en.
How can we remember? Look for short cuts!
All the plural adjectives end in -en. So do the dative and genitive cases!