What's the い in ない?

Question

I've heard that the non-past negative form ends in nai. However, other forms will aswell have na alone (na katta). Is there some meaning behind this い or is that how the form is?

Answer 1

In Standard Japanese, the negative particle (auxiliary) has been totally conflated with the adjective ない "not exist" in form. And in older language, the form corresponds to ない was なき (naki), so it was sharing the last -k with other forms. It seems somewhat irregular only after the k-sound dropped before i. Some dialects drop k before other vowels, which makes conjugation more complex (See 1, 2). It is not uncommon that velar consonants disappear in the middle of word.

English sail < Old English segel, segl (cf. German Segel)
English royal < French royal < Latin regalis

By the way, the perfect (past) form なかった (nakatta) is originally a contraction of なく (naku) + あった (atta "was").

Answer 2

That's how the form is. い-Adjectives in Japanese are a bit like verbs - they have many (but not all) of the same inflections that verbs do, and often those inflections mean the い gets replaced with something else. This is just like the final -う or -る of verbs getting replaced when they get inflected.

Here are some examples with 食{た}べ (eat) and 美味{おい}しい (tasty):

• simple past: 食べた, 美味しかった
• conditional (tara) form: 食べたら, 美味しかったら
• "hypothetical" (ba) form: 食べれば, 美味しければ
• te-form: 食べて, 美味しくて (note: 美味しくて does not mean "be delicious!")
• negative: 食べない, 美味しくない

And relevant to your example:

• past negative: 食べなかった, 美味しくなかった

So ～なかった is just the past version of ～ない.