So I'm trying to understand Genki's preferred past negative form of です, that
is: じゃなかったです. I'm confused by the なかった.

As I understand it from [this
source](https://www.japanesewithanime.com/2019/11/jodoushi.html), this is a
combination of two 助動詞: the 連用形 form of ない continued with た. However,
[wiktionary](https://en.wiktionary.org/wiki/%E3%81%AA%E3%81%84#Japanese) lists
the 連用形 form of ない as なく.

なくた sounds a lot like なかった, so it seems reasonable to assume this is a vowel
shift to avoid the awkward transition between U to T in KUTA, but, since I'm a
beginner, I wanted to make sure - are there other grammatical processes in
place that I've missed?

Which leads me to the following overall theory:

There are five "lexemes" in play here

  * じゃ -> DEWA relating the verb phrase to the rest of the sentence
  * なか -> NAI informing of negation
  * った -> TE informing of past
  * です -> DE relating the 助動詞 to ARIMASU -> informing of "existence"

these are combined with somewhat irregular conjugation process which involves
shifting verb roots and auxillary verbs into one of a few base forms and then
combining them. I assume the grammar of this process is more descriptive than
prescriptive, and the speaker typically memorizes a list of common
conjugations and common exceptions, rarely applying the rules to create new
verb constructs. Or is there a formal grammar to this process described
somewhere?
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2021-01-23T06:24:04.820
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question
verbs auxiliaries
Where does the か in なかった come from?