Only the final い changes. Because the adjective already holds the tense, “was expensive” is 高かったです, never 高いでした.
Short answer: most Japanese adjectives end in い, and they work like verbs: they carry their own tense and negative. 高い alone means “is expensive”; change い to くない for “is not”, to かった for “was”, and to くなかった for “was not”. Put one straight before a noun to describe it (高い車, an expensive car), and add です at the end only for politeness.
Twenty common い-adjectives
Press each to hear it. Several come in opposite pairs, which makes them easier to learn together.
The four forms, plain and polite
Every い-adjective follows this table. The polite column simply adds です.
Form
Meaning
Polite
takai
is expensive
高いです
takakunai
is not expensive
高くないです
takakatta
was expensive
高かったです
takakunakatta
was not expensive
高くなかったです
いい: the one irregular adjective
いい (good) comes from an older word, よい, and every form except the plain present uses that root. かっこいい (cool) follows the same pattern.
Before a noun
Put the adjective directly before the noun, in any form. No extra word is needed, which is the main difference from な-adjectives.
Linking adjectives: くて
To join two adjectives (“cheap and tasty”), change the first い to くて. This is the adjective’s て form.
Heya wa hirokute akarukatta desu.“The room was big and bright.”Why:The past goes on the last adjective only; くて takes its tense from the end.
From adjective to adverb: く
Change い to く and the adjective describes a verb: 早い (early) becomes 早く (early, quickly). With なる (become) it means “get…”: 寒くなる, “get cold”.
Adjectives that end in い but are not い-adjectives
A few common な-adjectives happen to end in い. They are written with kanji whose reading ends in い, or borrowed from Chinese. Learn them as exceptions; the な-adjectives page shows how they conjugate.
〜そう: it looks…
Drop the final い and add そう to say how something looks, before you have tried it. いい and ない have special forms with さ.
Form
Meaning
From
oishisō
looks delicious
美味しい
takasō
looks expensive
高い
muzukashisō
looks difficult
難しい
yosasō
looks good
いい (special)
nasasō
seems there isn’t
ない (special)
Too much: 〜すぎる
Drop い and add すぎる (to exceed). The result is a Group 2 verb.
Becoming and making: くなる, くする
く + なる means something becomes that way by itself; く + する means someone makes it that way.
寒くsamukucoldなりましたnarimashitabecame
Samuku narimashita.“It’s got cold.”Why:くなる: a change by itself.