Catalyzes the reduction of dihydromonapterin to tetrahydromonapterin. Also has lower activity with dihydrofolate. (6S)-5,6,7,8-tetrahydrofolate + NADP(+) = 7,8-dihydrofolate + H(+) + NADPH 7,8-dihydromonapterin + H(+) + NADPH = 5,6,7,8-tetrahydromonapterin + NADP(+) Belongs to the short-chain dehydrogenases/reductases (SDR) family. FolM subfamily.