I adjective ku/katta


I am wondering if the ku/katta conjugation for i adjectives has (had) a
meaning by itself.

For example tanoshii becomes tanoshiku. In the past tense then tanoshikatta.
Clearly the tta is the indicator for the past tense. So my question is: is or
was the ku/katta for i adjectives a verb ? Where does it come from ? And what
is its meaning (beside the one learned in textbooks). My japanese teacher also
wondered but she mentioned that ka, ki, ku, ke, ko were used in old japanese
to indicate past or so. I have heard once that i adjectives have the desu/da
already built in so maybe ku/katta is some variation of da/datta?




Japanese-as-a-second-language learners generally remember 楽しかった and so on as a
form of 楽しい. However, in the "traditional" Japanese grammar, た/だ is indeed a
standalone word categorized as a 助動詞 (auxiliary verb).

  * [Auxiliary verbs in Japanese](https://japanese.stackexchange.com/q/63351/5010)
  * [Why Japanese verb has so many forms?](https://japanese.stackexchange.com/q/61902/5010)

So yes, た in 見た, た in 簡単だった, and た in 楽しかった are exactly the same 助動詞 for the
past tense. For example,
[this](https://dictionary.goo.ne.jp/word/%E3%81%9F/#jn-132183) is the entry
for た in a monolingual dictionary, and it clearly says it's an auxiliary verb
that signifies the past tense.

く and かっ are considered as part of the "true" conjugation forms of
i-adjectives (namely [連用形](https://japanese.stackexchange.com/a/65953/5010)).
They are not regarded as a word, and they don't have any intrinsic meaning on
their own. The following table is the conjugation chart of i-adjectives
described in Japanese Wikipedia.

[![形容詞の活用表](https://i.stack.imgur.com/1J3sG.png)](https://i.stack.imgur.com/1J3sG.png)

