い-adjectives

In Japanese there are two types of adjectives (い-adjectives and な-adjectives). い adjectives end in ai, ii, ui or oi.

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ちいさい おおきい あつい おもしろい

 

い-adjectives conjugate in a different way than な-adjectives and nouns.

Affirmative form, present

These are used in a polite way because です which indicates polite speech is placed after the adjectives.
Concerning the affirmative form in present, い-adjectives, な-adjectives and nouns conjugate in the same way.

Negative form, present

The final い is replaced with and ありません is added to make the present tense in the negative form.
ちいさ → ちいさくありません (The final い is replaced with く)

Affirmative form, past

The final い is replaced with かった which expresses the past.
ちいさ → ちいさかった ちいさかった → ちいさかったです
As かった doesn’t indicate polite speech, です is added to express politeness.

Negative form, past

The final い is replaced with and ありませんでした is added to make the past tense in the negative form.
ちいさくありません → ちいさくありませんでした