First you write the "molecular" equation, then the ionic equation, and finally the net ionic equation.
Explanation:
You can read about these equations at
Here are the steps for your reaction.
1. Write the molecular equation
We write the equation as if all the reactants were molecules.
2HNO₃(aq) + Mg(OH)₂(s) → 2H₂O(l) + Mg(NO₃)₂(aq
2. Write the ionic equation
We write all the soluble strong electrolytes as ions. H₂O is a weak electrolyte, so we write it as a molecule. Mg(OH)₂ is insoluble, so we write it as a molecule as well.
2H⁺(aq) + 2NO₃⁻(aq) + Mg(OH)₂(s) → 2H₂O(l) + Mg²⁺(aq) + 2NO₃⁻(aq)
3. Write the net ionic equation
Here, we cancel all species that appear on both sides of the equation. We cancel the NO₃⁻ ions.
The net ionic equation is
2H⁺(aq) + Mg(OH)₂(s) → 2H₂O(l) + Mg²⁺(aq)
