The crucial point in both formal and natural language is that the negation of a universal statement is an existential statement, and vice versa; specifically, "not everybody" is equivalent to "there exists someone who does not" and "nobody" is equivalent to "everybody does not" although English grammar permits two interpretations of the last expression; I use it to mean "for everybody holds that they do not...".
_While doing the exercises, also keep in mind that the negation of a conjunction is a disjunction and vice versa; e.g., not (A and B) is equivalent to ((not A) or (not B))._