I'm going to have to knuckle down to some serious study.

我得开始认认真真地学习了。

He would not knuckle down under their pressure.

他不会在他们的压力下屈服的。

You 'll really have to knuckle down if you want to pass the examination.

如果想通过考试，你确实应专心学习。

He managed to knuckle down to his lessons long enough to pass his examination.

他在离考试还有足够长时间的时候就开始认真学习，从而通过了考试。

As for Ronaldinho's fitness, if he doesn't move I am sure that he will knuckle down and train hard to get back to his previous levels.

鉴于罗纳尔迪尼奥的健康状况，如果他不转会，我确定他能够一扫阴霾并且努力训练找回自己的最佳状态。