knuckle down to(v.+adv.+prep.)
〈非正〉开始努力做… begin to work hard at sth
▲knuckle down to sth
He knuckled down to his research.他开始致力于研究。
I'm going to have to knuckle down to some serious study.我得开始认认真真地学习了。

knuckle under to(v.+adv.+prep.)
〈非正〉向…认输；屈服于 accept sb else's authority
▲knuckle under to sb/sth
He refused to knuckle under to any pressure.他拒绝屈服于任何压力。