Once will have to do. You can go just this once. 必须去做一次，你只去这一回一次。 Once is enough for me. 对我来说一次就够了。 . 发生一次；一次： once和assoonas用法比较 2021/1/13 1 ；以前的 a once friend 从前的一个朋友 the once capital of the nation. 该国以前的首都 once和assoonas用法比较 2021/1/13 2 . (1).once作连词时，意为“一旦……就……”，用于连接时间状语从句，相当于as soon as。如： Once he goes, we can clean ，我们就能清理 (2).一旦,一经...便 Once you get into a bad habit, you'll find it hard to get out of it. 一旦染上坏习惯,想改掉就难了 Once you begin you must , 便不可停下来。 once和assoonas用法比较 2021/1/13 3 . (1). 也可意为“一次”的意思。 once a I see them once every two months. 我每两个月与他们见一次面。 (2).意为“曾经；一度；从前”。它是一个不确定的时间副词，如： a once powerful nation曾经很强盛的国家 Once known, his face is never ，他的面容让人永生难忘 Once he lived in America, but now he lives in ，但现在他生活在英国。 once和assoonas用法比较 2021/1/13 4 (3).乘以一; (亲属间行辈上)小一辈; a cousin once removed 堂[表]兄弟[姊妹]的孩子 Once two is two. 一二得二。 (4).用于否定句，一次也(不); 完全(不) I couldn't once understand what he 。 once和assoonas用法比较 2021/1/13 5 注意： I once went to Shanghai. 我曾经去过上海。 I went to Shanghai once. 我去过上海一次。 once和assoonas用法比较 2021/1/13 6 Once的特殊用法 （1）“Once+过去分词”做后置定语，可理解为“if once”，具有条件的含义，例如 This rule once learned will never be forgotten. If once this rule is learned it will never be forgotten. A fish once taken out of the water soon dies. If once a fish is taken out of the water it will die once和assoonas用法比较 2021/1/13 7 (2)Once连接一个省略句（省略与主句相同的主语或it及相应的be） Once combined,nitrogen is no longer chemically inert gas . (once nitrogen is combined，it is not chemically inert any longer.) The magnesium has to be warmed to start a rapid reaction, but once started, large quantities of heat and light energy are liberated. (~~~but once it is started,~~~)镁必须予以加热才能迅速反应，一旦反应开始就会放出大量的热和光。 氮气一经化合就不再是惰性气体 once和assoonas用法比较 2021/1/13 8 Once的短语 once- once- all at once突然; 同时 at
