"Know sb或sth backwards"是什么意思?

know sb/sth backwards 对…了如指掌（或倒背如流）；把…背得滚瓜烂熟

to know sb/sth extremely well

•She must know the play backwards by now↗. So don’t worry about the coming show. ↘

她现在对这个剧本肯定是倒背如流。所以，你不用担心即将到来的表演。

give sb. a knowing smile 对着某人会意的笑

A knowing gesture or remark is one that shows that you understand something, for example the way that someone is feeling or what they really mean, even though it has not been mentioned directly.

•Ron gave Alice↗ a knowing smile / after she finished her speech. ↘

在Alice讲话结束的时候，罗恩向她报以一个会意的微笑。

sb. know best (about) 某人最具权威性；某人最了解

•As the person↗ who designed the entire system↗, I think I know best / about how to protect our users. ↘

作为设计整个系统的人，我认为我最了解如何保护我们的用户。