木已成舟（an accomplished fact）<br>
=fait accompli  <br>
=The die is cast<br>
=What is done cannot be undone.
对牛弹琴<br>
=be like talking to a brick wall<br>
=preach to deaf ears<br>
=cast pearls before swine
塞翁失马，焉知祸福<br>
=a blessing in disguise<br>
=a mixed blessing<br>
=a loss may turn out to be a gain