Let $M = \left[ \begin{array} { l l } { 1 } & { 2 } \\ { 3 } & { 0 } \end{array} \right]$ and $N = \left[ \begin{array} { l l } { a } & { 4 } \\ { c } & { d } \end{array} \right]$. In some cases $MN = NM$. Find these conditions.