Only when a reservoir is built here can we solve our irrigation problem. 除非在这里修个水库,才能解决灌溉问题。
If you don't want people to know, you'd better not do it. 若要人不知,除非己莫为。
牛津高阶第八版
美国韦氏词典
Dictionary.com
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维基百科 (自由的百科全书)