She won't go to bed _______ her grandma tells her a story.这里

unless引导的是条件状语从句, 此句刚好符合"主句用一般将来时, 条状从用一般现在时"这条规则.表达的是一个还未发生的动作.
而until引导的是时间状语从句, 虽然也符合"主句用一般将来时, 时状从用一般现在时"这条规则,但.not...until句型, 主要是强调过去发生的动作, 主、从句都要用一般过去时.
即： She didn't go to bed until her grandma told her a story.
She won't go to bed unless her grandma tells her a story.

until（条件句）后用现在完成时，∵原句用了tells ∴用unless

unless
She won't go to bed if her grandmother doesn't tell her a story.