undo
/ʌnˈdu:; ʌn`du/ v
(pt undid / ʌnˈdɪd; ʌn`dɪd/, pp undone / ʌnˈdʌn; ʌn`dʌn/) [Tn]
1 untie or unfasten (knots, buttons, etc); open (a parcel, an envelope, etc) 解开, 松开（结、 钮扣等）; 打开, 拆开（包裹、 信封等）
My zip has come undone. 我的拉锁开了.
I can't undo my shoelaces. 我解不开鞋带.
undo (ie unravel) some knitting 拆开编结物. Cf 参看 do up (do2), do sth up.
2 destroy the effect of (sth); cancel 使（某事物）无效;取消; 废除
He undid most of the good work of his predecessor. 他把前任的大部分业绩毁掉了.
What is done cannot be undone. 已做的事是既成事实.