Na-adjectives with です

With a na-adjective, keep the adjective and change the ending, just as you do with a noun. Compare 学生がくせい です / 学生がくせい じゃ ない です with 元気げんき です / 元気げんき じゃ ない です.

The name comes from the な used directly before a noun: 有名ゆうめいな 先生せんせい means “a famous teacher.” When the adjective is followed by です, leave out な: 有名ゆうめい です.

Do not classify an adjective just by its last sound. きれい (“beautiful” or “clean”) ends in い but is a na-adjective, as is 有名ゆうめい. The い is part of the word, not the ending you remove for an i-adjective. Use きれい じゃ ない, not きれくない. Learn the adjective group along with each new word.