you never know：很难说，很难预料
    e.g. You never know, she might change her mind.
    很难说，她或许会改变想法。

    you never can tell：很难说，没法预料
    e.g. Who knows what will happen to Peter and me in the future, you never can tell.
    谁知道彼得和我将来会怎么样呢？永远无法预料。

    know what’s waht：懂行，老练
    e.g. Xiaojiang’s been in the business for 10 years, she knows what’s what.
    小酱从事这一行业已有10年了，她很懂行。

    know sth inside out：精通；对…了如指掌
    e.g. He knows the system inside out.
    他对该体系了如指掌。

    know sth back to front：熟谙；对…了如指掌
    e.g. She knows her part in the play back to front.
    她熟谙自己在剧中的角色。

    know-it-all：自以为无所不知的人，英式中常用 know-all，讽刺人
    e.g. She is such a know-it-all, she always thinks she is right.
    她自以为是个百事通，总认为自己是对。

    know all the answers：自以为比别人知道得多，自恃聪明
    e.g. Allison thinks shw knows all the answers, but she’s clueless about a lot of things.
    艾莉森自以为比别人懂得多，但她对很多事情一无所知。

    know your place：安分守己；安于现状
    I just get on with my job and do as I’m told, I know my place.
    我只是按吩咐做好我的工作，我很守本分。