jmc

This subtraction is fairly straightforward: we just subtract the respective digits. No borrowing occurs: \begin{array}{c@{}c@{\;}c@{}c} & & 5 & 8_9 \\ &- & 1 & 8_9 \\ \cline{2-4} & & 4 & 0_9 \\ \end{array} Thus, the answer is $\boxed{40_9}.$