Show that it is not the case that if ⊨ ¬(A ∧ B) then ⊨ ¬A or ⊨ ¬B.
Consider the formula ¬(p∧¬p).
Replace A with p and B with ¬p.
Validity: this is defined as truth preservation over all worlds of all interpretations. The intuitionist logical consequence is |=I