"You learn nothing."
Translation:Du lernst nichts.
Ihr lernen nichts. Should be correct.
It is not.
both subject and verb are in agreement.
They are not.
ihr requires verb forms ending in -t -- thus, ihr lernt.
ihr lernen is just as wrong as "I learns".
(Many people have the mistaken belief that there's such a thing as "a plural verb form" in German that applies to all plural subjects. That's just as untrue as believing that there's "a singular verb form" in English that would allow you to say "I learns" because "he learns" is correct and "I" is also singular just like "he".)
Sie lernst nie is not correct, because the verb form lernst with -st belongs to du, not to sie.
Sie lernt nie is grammatically correct but means "She never learns", not "She learns nothing".
And it can't mean "You learn nothing", because Sie (formal you) takes the verb forms of sie "they", not of sie "she", i.e. Sie lernen nichts.