一天one day
毁于一旦
be destroyed in a day
忽然suddenly
相处三年，一旦别离，怎么能不惋惜？
After three years together, if you suddenly had to part, would you not be sad?
如果if
一旦有可能
if possible
一旦时机成熟
if the opportunity presents itself
一旦实现
if it happens
一旦有事，大家会帮助你的。
If you have a problem, everyone will help you.
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