In Japanese there are two types of adjectives, na keiyoushi (な形容詞) and i keyoushi (い形容詞). Unlike verbs, Japanese adjectives are more easy to tell the difference between the two groups. There are also some adjectives that are both types of adjectives.
i keiyoushi are adjectives in Japanese that have to end in i before they modify a noun. Often adjectives are written in kanji, if an adjective has the letter i (い) after the kanji, it is an i-keiyoushi.
Tall building = takai biru (高いビル)
To conjugate a i-adjective into the negative form, you remove i on the end and add kunai (くない).
Not tall building = takakunai biru (高くないビル)
Converting an adjective into the past and negative past form is also easy. To change it into past form you remove i and add katta (かった). To change the adjective into the negative past form you remove i and add kunakatta (くなかった).
Was tall building = takakatta biru (高かったビル)
Was not tall building = takakunakatta biru (高くなかったビル)
na keiyoushi are adjectives in Japanese that have to end in na before they modify a noun. If the adjective is written in kanji and it is not followed by the letter i (い), then it is a na-keiyoushi. This doesn't mean that the word can not end with the i sound, It means that there isn't a hiragana character after the kanji that is i.
However, there is one adjective that ends in a hiragana i that is a na adjective. Kirai (嫌い) which means, hateful or dislike. Some adjectives are commonly written in hiragana, so it can be hard to tell which kind of adjective they are. One example is Kirei (きれい), which means clean, neat or pretty.
To modify a noun directly, you need to add the character na (な) after the adjective before the noun its modifying.
She is quiet person = kanojo wa shizuka na hito desu
She is quiet = kanojo wa shizuka desu
•Notice the difference between these two sentences. The first one modifies the noun directly. The second sentence does not, so there is no need to add na.
To conjugate a na-adjective into the negative form, add ja nai (じゃない) to the end of the adjective.
Not quiet person = shizuka ja nai hito (静かじゃない人)
Converting an adjective into the past and negative past form is also easy. All you do is add the ending datta (だった) for the past, and add ja nakatta (じゃなかった) for negative past.
Was quiet = shizuka datta (静かだった)
Wasn't quiet = shizuka ja nakatta (静かじゃなかった)