The best Williamson route to tert-butyl methyl ether, $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{O}-\mathrm{CH}{}_3$, is

The alkyl halide must be primary (or methyl) so that SN2 wins over E2. Sodium tert-butoxide with $\mathrm{CH}{}_3\mathrm{I}$ gives the ether, while $\mathrm{CH}{}_3\mathrm{ONa}$ with tert-butyl bromide gives 2-methylpropene by elimination.