Reaction of phosphorous acid (H3PO3) and potassium hydroxide (KOH) For the reaction of phosphorous acid ($\ce{H3PO3}$) and potassium
hydroxide ($\ce{KOH}$), write (a) the balanced molecular equation
and (b) the net ionic equation. Note that phosphorous acid is
a diprotic acid.
$$\ce{H3PO3(aq) + 2KOH (aq) -&gt; 2 H2O(l) + K2HPO3 (aq)} \tag{a}$$
$$\ce{H3PO3(aq) + 2 OH-(aq) -&gt; 2 H2O(l) + HPO3^2-} \tag{b}$$

The answers are above. Can somebody explain why it is $\ce{K2HPO3}$ in (a)? I am familiar with monoprotic acid, and this is my first encounter of diprotic acid. Thanks.
You have made the assumption that all the hydrogens are attached to oxygens. This is not a good assumption; one of the hydrogens is bonded directly to phosphorus and is not removed by OH-.
A better description of phosphorous acid is HPO(OH)2 with one oxygen doubly bonded to phosphorus  wikipedia