keep down:1）有镇压的意思 例句：The government keeps down its people.
2)有控制的意思,比如控制声音 keep the volume down
3）控制不呕吐的意思 例：It is difficult to keep anything down when you have the flu.（摘自wikipedia）
4）猫着腰,不站起来 例：you'd better keep down or else they will see you
keep under:通常不用,基本是 keep sth under sth,把什么放到什么下面的意思,比如keep your phone under your
desk把手机放到桌子下面
