Write the net ionic equation for the hydrolysis of hydrocyanic acid, HCN(aq)?

The equation is HCN(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CN⁻(aq)
Or simply, HCN(aq) ⇌ H⁺(aq) + CN⁻(aq)

Hydrolysis is adding water. The reaction is:
HCN + H2O -> CN- + H3O+
To write the net ionic equation, first write the total ionic equation.
So I will split up all the molecules into individual ions.

H(+) + CN(-) + H2O -> CN(-) + H3O(+)

To write the net ionic equation, I will eliminate all spectator ions. Spectator ions are ones that don't participate in the reaction and appear on both sides of the equation.

The spectator ion is: CN(-).
The remaining reaction is:

H(+) + H2O -> H3O(+)

That is your answer.

The hydrolysis of hydrocyanic acid would yield ammonium formate: HCN + 2H2O --> HCOO- NH4+