When it comes to
当一───When one;提到───mention
's not unusual for the police to close ranks when one of their officers is being investigated.───一位警员受到调查时，警察抱团的事并不罕见。
one of my dogs died, the other one sorrowed.───我的一条狗死了，另一条很悲伤。
The helicopter crash landed when one of its two engines cut out.───直升飞机两个发动机中的一个突然熄火时，直升机撞地着陆。
A smile displays beautiful teeth when one has them; but she did not smile.───人可以在笑时露出美丽的牙齿，但是她一点也不笑。
To her great surprise, she recognized the book when one of our fellow disciples gave her a sample booklet.───当收到同修送给她样书时，她认出这就是她梦见的绿书，她很惊喜。
Nevertheless, the time when one quietens down and thinks back, one would miss the cozy racket that used to be so annoying.───可是，一个人安静下来时会觉得，曾经厌烦的那些嘈杂回想起来很温情很怀念。
When one player behaves disgracefully, it reflects on the whole team.
He was obliged to resign when one of his own aides was implicated in a financial scandal.
When one is about to act, one must reason first.
When one loves one's art no service seems too hard .
When one door of happiness closes, another opens; but often we look so long at the closed door that we do not see the one which has been opened for us.