Adjectives in Japanese are very different from English adjectives. Although they do serve to describe a noun, Japanese adjectives, like verbs, have a present and past form! That is, you change the adjective depending on whether you are saying “The book is interesting” or “The book was interesting.”

Adjectives end in one of ii, oiai、or ui. You use the dictionary form to directly modify a word. The adjective precedes the word, just as it does in English.

ookiihon (big book)
osoijitenshiya (slow bicycle)
furuiinu (old dog)
akaiitigo (red strawberry)

To create a present tense sentence, you use desu at the end of the sentence.

honhaookiidesu (The book is big.)
jitenshiyahaosoidesu (The bicycle is slow.)
inuhafuruidesu (The dog is old.)
itigohaakaidesu (The strawberry is red.)

To change to past tense, drop the i and add ka*ta.

honhaookika*tadesu (The book was big.)
jitenshiyahaosoka*tadesu (The bicycle was slow.)
inuhafuruka*tadesu (The dog was old.)
itigohaakaka*tadesu (The strawberry was red.)

Negative Sentences

These are a bit more complicated. To make a present negative, drop i and add kunai.

honhaookikunaidesu (The book is not big.)
jitenshiyahaosokunaidesu (The bicycle is not slow.)
inuhafurukunaidesu (The dog is not old.)
toufuhaakakunaidesu (The tofu is not red.)

For past tense—are you ready for this?—drop the i and add this big mouthful of syllables:
kuarimasendeshita or

honhaookikuarimasendeshita (The book was not big.)
jitenshiyahaosokuarimasendeshita (The bicycle was not slow.)
inuhafurukunaka*tadesu (The dog was not old.)
toufuhaakakunaka*tadesu (The tofu was not red.)