These behave like nouns and need な when directly modifying nouns.

Form	Rule	Romaji
Present Positive (casual)	add だ	kireida
Present Positive (formal)	add です	kirei ~~desu~~
Present Negative (casual)	add じゃない	kirei ~~janai~~
Present Negative (formal)	add じゃありません	kirei ~~ja arimasen~~
Past Positive (casual)	add だった	kirei~~datta~~
Past Positive (formal)	add でした	kirei ~~deshita~~
Past Negative (casual)	add じゃなかった	kirei ~~janakatta~~
Past Negative (formal)	add じゃありませんでした	kirei ~~ja arimasen deshita~~
Adverbial	add に	kireini
Te-form	add で	kireide