Can Cyanide ion substitute methoxide?
You need to reckon with how strong a base the nucleophile is compared with what you are trying to displace.  Only if the nucleophile is the stronger base can you expect the reaction to go based on thermodynamics.

Thus, both of the following reactions can go:

$\ce{RCl + OCH3^- -&gt; ROCH3  +Cl^-}$

$\ce{RCl + CN^- -&gt; RCN  +Cl^-}$

because both nucleophiles are stronger bases towards carbon than the chloride ion that would be displaced.  You can then say that the second reaction will go faster with a better (kinetic) nucleophile.  But if you have

$\ce{ROCH3 + CN^- -&gt; RCN  +OCH3^-}$

where you are trying to displace the strongly basic methoxide ion rather than adding it as a nucleophile like the first reaction above, then cyanide ion is too weakly basic to push through.  Cyanide ion can be a good nucleophile only when displacing a weaker base or adding to an appropriate unsaturated compound (such as a carbonyl compound) without displacement.

Try it now.