bygone
adjective [ before noun ] uk/ˈbaɪ.ɡɒn/ us/ˈbaɪ.ɡɑːn/
过去的，以往的
belonging to or happening in a past time
a bygone era 过去的时代
bygone
noun [ plural ] uk/ˈbaɪ.ɡɒn/ us/ˈbaɪ.ɡɑːn/
let bygones be bygones （尤指既往不咎）过去的事就让它过去吧
used to tell someone that they should forget about unpleasant things that happened in the past, and especially to forgive and forget something bad that someone has done to them
Just let bygones be bygones and be friends again. 过去的事就别再提了，重归于好吧。
Why can't you let bygones be bygones and put it all behind you?
My mother thinks I should let bygones be bygones but I'm struggling with it.
After all these years, surely it's time to let bygones be bygones.
If she's willing, I'm willing to let bygones be bygones.