back then, did Adam and Eve know what death is?
let's see, they are vegetarians, so they have to eat plants, so they have to have some idea of what it's like for plants to be bitten, and eaten. When fruits get's plucked from trees, hence, separated from the vine, branch, or roots.
So they have an idea what it might mean to be separated from the source of life, they can guess that death involves being chewed or grind on, from their teeth.
Adam and Eve have an idea what the consequence might be, yet, the desire to sin is somehow bad enough that they ignore the consequence and sinned