know sth. backwards and forwards 洞悉某事物

After 30 years in the business, she knows it backwards and forwards.

Be rusty 某种技能由于缺乏使用而变得不够熟练

Be proficient in sth. 掌握得非常纯熟

Be out of sb.'s depth 超出了某人的能力范围

testing ground 试验场