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1、once的用法,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。,1.n. 发生一次；一次：,2.adj.曾经的；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,3.conj.,(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 up.他一走，我们就能清理 (2).
2、一旦,一经.便Once you get into a bad habit, youll 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 continue.你一旦开始, 便不可停下来。,4.adv.,(1). 也可意为“一次”的意思。 once a day.一天一次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
3、, his face is never forgotten.曾经认识，他的面容让人永生难忘 Once he lived in America, but now he lives in England.他曾经生活在美国，但现在他生活在英国。,(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 couldnt once understand what he meant.我完全不懂他的意思。,注意： I once went to Shanghai.
4、 我曾经去过上海。 I went to Shanghai once. 我去过上海一次。,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 d
5、ie,(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. (b
6、ut 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的短语,once-forbidden.一次禁戒的 once-through.强制循环 all at once突然; 同时 at once立刻, 马上; 同时 every once in a while美偶而 for this once (=for once, just for once)就这一次; 破例一回 if once (=when once)一旦 more than once不止一次, 多次 not once一次也不 once and aga
7、in一再, 再三,once and away一劳永逸; 限此一次; 断然, 爽快 once for all一劳永逸; 永久地; 彻底地 once in a while有时, 间或, 偶而 once more再一次 once again再一次 once or twice一两次; 有时, 偶尔 once over lightly从头到尾马马虎虎地 once too often又(多了)一次,every（形容词） 与once,The Olympic games are held every four years. The Olympic games are held once four years.
8、 奥运会四年举办一次,Once与as soon as,引导的是状语从句时，主句一般将来时，从句用一般现在时表示将来 Physics is easy to learn once you understand the rules. 一旦你理解了规则，物理就不难学了. I will tell him the news as soon as he comes back. Well let you know as soon as we come to a decision. 我们一有决定，马上通知你.,once引导时间和条件状语连词 as soon as引导是时间状语连词 I will buy the whole world once I get rich. 我 一 旦 有 钱 了 ， 就 要 把 整 个 世 界 买 下 来 。 I will buy the whole world as soon as I get rich. 我 发 财 的 那 一 刻 就 把 整 个 世 界 买 下 .,