# Why ethers only undergo cleavage with strong acids and not with weaker acids?

Basically because the $C - O$ bond is strong........
As far as anyone knows the oxygen, as the most basic site of the ether, $R - O - R$, is protonated by an acid, $H X$, to give $R - {O}^{+} H - R$ and ${X}^{-}$. This weakens the $C - O$ bond such that it may cleaved by (say) a water molecule to give $2 \times R O H$.
The stronger the acid, $H X$, the weaker the $H - X$ bond, and the more likely it is to cause hydrolysis of the ether moiety.