Why is the reaction of calcium oxide with sulfur dioxide to form calcium sulfite not a combination redox reaction?
To determine whether a reaction is a redox reaction, you first have to find the oxidation number of each element in the reactants and products. You do this by either counting the number of electrons in each bond and assigning them to the most electronegative atom in the bond, or by memorizing some rules and applying them. When you do that for this reaction, you should get the following:

\begin{align}
  \ce{   &amp;&amp;CaO + SO2 &amp;-&gt; CaSO3}\\
  \ce{Ca:&amp;&amp; +2 &amp;-&gt; +2}\\
  \ce{O: &amp;&amp; -2, -2 &amp;-&gt; -2}\\
  \ce{S: &amp;&amp; +4 &amp;-&gt; +4}
\end{align}

Since the oxidation number hasn't changed for any element, no reduction or oxidation has taken place, and this is not a redox reaction.