Methanal reacts with $\mathrm{CH}{}_3\mathrm{MgBr}$ in dry ether and the adduct is then treated with dilute acid. The product is

A Grignard reagent adds to methanal to give a primary alkoxide with one more carbon than the Grignard alkyl group, $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{OMgBr}$. Hydrolysis gives $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{OH}$. An aldehyde other than methanal would give a secondary alcohol and a ketone would give a tertiary alcohol.