In German, adjectives before a noun get an ending. The ending depends on what comes BEFORE the adjective:
1. After definite article (der, die, das) β weak declension
2. After indefinite article (ein, eine, kein) β mixed declension
3. Without article β strong declension
The basic rule: something must show the gender!
If the article shows gender β adjective "relaxes" to -e/-en