。
Mom tells me not to study too hard.
Literal
Mom [subject-が] too-much eagerly study don't [quotative-と] says [explanatory-のです].
The quoted imperative 勉強するな uses the plain negative imperative ~な, a blunt 'don't X' suffixed directly to the dictionary form. It's rough-sounding as a direct command but softens inside a quotation. あまり + negative = 'not too much.' 一生懸命 ('with all one's might, desperately') is a yoji-jukugo (four-character compound) meaning 'earnestly, with full effort.' のです explains/softens the whole report.