/jp/ - Otaku Culture

Hello /djt/, newbie here. I'm watching Cure Dolly's videos.
She says that to say "Sakura did not run", you'd say:
サクラがはしらなかった
サクラが = Sakura
はしら = runs
なかった = did not
My question is, why is "runs" not put in the past tense, and instead the word for "not exist" is put into past tense? Because to say "Sakura ran" you'd say
サクラがはしった
サクラが = Sakura
はしった = ran
I don't think she touched on that. I'm assuming there's some Japanese rule about this that I didn't pick up on.
Thanks in advance.

Beginner here, watching Cure Dolly's playlist for learning Japanese. Is it normal to be completely lost on the adjective lesson?
https://youtu.be/iyVZlaEqU24
The second half is easier to understand but the first half, I don't know. Nouns as adjectives? Verbs as adjectives?
I'll just have to give it another watch. I'll do the worksheet too.