。
You mustn't forget to talk to him tomorrow.
Literal
Tomorrow him [with-と] talk [object-を] do [nominalizer-の] [object-を] forget [must-not-てはいけません] [emphasis-よ].
~てはいけません is the polite prohibition — 'you must not X.' の nominalizes 話をする into the object of 忘れる. The sentence-final よ adds personal insistence, turning this from a neutral rule into pointed advice.