名詞 bygone:

1. past events to be put aside
   同義詞：water under the bridge

形容詞 bygone:

1. well in the past; former
   同義詞：bypast, departed, foregone, gone

1. His talk was evocative of the bygone days.
   他的談話令人回憶起往昔的時日。
2. The two boys agreed to let bygone be bygone and make friend again.
   兩個孩子決定捐棄前嫌，重新做朋友。
3. Our family was very poor in the bygone age.
   我家過去很窮。
4. Let bygones be bygones.
   過去的，就讓它過去吧。

by.gone

adj.（形容詞）
Gone by; past:
過去的；以前的：
例句：
bygone days.
過去的日子

n.（名詞）
One, especially a grievance, that is past:
過去的事：過去的事，尤指委屈：
例句：
Let bygones be bygones.
過去的事就讓它過去吧