Negatives problem

Question

According to Cure Dolly's guide,~ない is an adjective/auxiliary adjective used for describing state of non-existence/not doing. When we use an adjective, we have to use が because a subject state is being described. We use a verb whenever we describing a state when something exist or being done:

猫がいる。 桜が走る。

And an adjective when something does not exist or not being done:

猫がいない。 桜が走らない。

For example, when auxiliary ~たい is used, we're using が, because a verb becomes an　adjective.

車が持ちたい。

Why don't we use が with negatives while they're in fact adjectives too?

車が持ちたい。

それをしない。(Never heard of それがしない.)

Answer 1

You could think it's something like this:

[私が]それをしない
I won't exist as doing that

Grammatically, 私がない is the sentence at its core. それをし just modifies ない. It describes the particular non-existence 私 does.

それがしない
That won't exist as doing (it)

The last one is pretty odd. No wonder it is never used. It doesn't mean "This is not done" or "This will not be done".

---

Lesson 88 expands on the issue of を and が in たい sentences. I will briefly summarize it.

Using を instead of が gives the idea of a non-impulse desire. それを分かる. さくらを助けたい. パンが食べたい.

それを食べたい

たい is an adjective meaning "desire-inducing" and it is modified by それをたべ. The subject might not 私 but the thing/situation that induces the desire to それをたべ. For example, "the fact of not having eaten それ for a long time" may be a subject.

Answer 2

It is better to consider VERB〜ない forms to still be syntactically verbs, which only conjugate like 〜い adjectives.

Also negative forms of verbs in Classical Japanese have more verbal conjugation (〜ず itself is inconjugable, but it participates in extended forms with あり, which undergo contraction; 〜ぬ was regular 四段活用 verb).

You can use particle を with desiderative VERB〜たい forms.

See also In what way is the negative form of a verb an adjective? and How does Japanese grammar think about verb negation?, が vs を in sentences of desire (-たい), Is it possible to use the 〜たい form with the particle が instead of を?.