There are 2 types of adjectives:
-i adjectives - adjectives that end in -i
-na adjectives - adjectives that add -na when placed before nouns
The -i adjectives change:
あつい atsui - (It's) hot || +i
Learn this and you can use all -i adjectives!
The -na adjectives don't change! But when placed before nouns they add a -na
げんき genki (healthy, active, fine) げんきな子 genki na ko (healthy child)