Na adjectives can't be declined like i adjectives can. So for instance, "omoshiroi" is an i adjective that means "interesting" and "omoshirokunai" means "not interesting". "Kirei" is a na adjective that means both "pretty" and "clean". To say "not pretty", you have to place the simple negative form of the copula after it. So "Kirei ja nai" and "kirei de wa nai" mean "not pretty". When you use a na adjective before a noun, the particle "na" has to come between it and the noun, so "kirei na hana" means "pretty flowers". An i adjective just comed before a noun as is. So "omoshiroi eiga" means "interesting movies". I adjectives have past tense declinations so "omoshirokatta" means "were interesting" and "omoshirokunakatta" means "weren't interesting". Na adjectives have to be followed by the copula in the past tense, so "kirei datta" means "were pretty" and "kirei ja nakatta" means "weren't pretty".