Adjective Conjugation – Past and Negative (～かった、～くない、～くなかった、etc.)

Two Types:
First of all, there are 2 classifications of adjectives in Japanese. The ‘i’ adjective and the ‘na’ adjective.
To see how to use the ‘na’ adjective click here.

‘i’ adjectives always end in an ‘i’, but never ‘ei’.
Common Examples:
Fun = tanoshii (楽しい), small = chiisai (小さい)*, cold = samui (寒い)

‘na’ adjectives don’t always end in ‘i’ and conjugate in a different way.
Common Examples:
Healthy = genki (元気), pretty = kirei (きれい), convenient = benri (べんり)

* The adjectives chiisai and Ookii -small and big- can be used also as ‘na’ adjective with no change in meaning (eg. chisai inu or chisa na inu – both mean small dog), though this is less common.

Conjugation Table (Polite):
‘i’ adjective                     ‘na’ adjective
Plain:                                    tanoshii desu                      genki desu
(J)                                             楽しいです                         元気です
Past (was):                          tanoshikatta desu              genki deshita
(J)                                             楽しかったです                 元気でした
Negative (isn’t):                tanoshikunai desu             genki dewa arimasen**
(J)                                             楽しくないです                 元気ではありません
Past Negative (wasn’t):  tanoshikunakatta desu     genki dewa arimasen deshita
(J)                                             楽しくなかったです         元気ではありませんでした

Casual:
For ‘i’ adjectives conjugations are the same; the only difference is you can drop the desu.

For ‘na’ adjectives, you can put the ‘desu’ and it’s forms a more casual way: ‘desu’ to ‘da’, ‘deshita’ to ‘datta’, ‘dewa arimasen’ to ‘dewa nai’, and finally, ‘dewa arimasen deshita’ to ‘dewa nakatta’.
NOTE: you can also use – instead of the forms of desu – jaarimasen and jaarimasen deshita ( じゃありません and じゃありませんでした) for polite, past and past negative adjectives. The casual forms are janai and janaikatta (じゃない and じゃなかった).

** ‘dewa nai’ is written ‘deha nai’ (ではない), but the ‘h’ is pronounced as a ‘w’, much like the particle ‘wa’ (は) which marks the topic. For clarity, this is the case for all its forms.

Adjective Exception: いい (Good), whenever its form is changed, become よい. So if you want to say ‘was good’ you say ‘yokatta desu’, and ‘not good’ would be ‘yokunai’ and so on…

To see a vocabulary list of basic Japanese adjectives click here.