How to convert ethanal to 2-hydroxy-3 butenoic acid?
Once you have got your acrolein from the aldol condensation of ethanal and formaldehyde you can react it with $\ce{NaCN, HCN}$ under kinetic control to get direct addition of the cyanide to the enone, forming a cyanohydrin[1]. The nitrile can then be hydrolysed in aqueous acid to yield the carboxylic acid.