一旦的英文：
in case (something happens)
once (something becomes true, then...)
in one day

参考例句：

Once a beggar, always a beggar.
一旦成为乞儿，就永无止境。

It is impossible to rehydrate a mucus globule once it has become dehydrated
一旦已经脱水，粘液不可能再水化。

Once the program is coded, the programmer must debug it.
程序一旦编成，程序员就必须调度它。

Pesticide persistence beyond the critical period for control leads to residue problems
农药一旦超过控制的临界期，就会导致残留问题。

Once we disabuse ourselves of the prejudice,we can cooperate with each other quite well.
一旦我们消除了这种偏见，就能很好地合作。

It is worth remembering that previous wills are nullified automatically upon marriage.
要记住的是，一旦结婚，原遗嘱自动失效。

If the truth is once elicited, they shall go scot-free.
一旦真相大白，他们也就脱清干系了。

Chicken Wrangler Zombie will release a flock of Zombie Chickens when damaged.
养鸡僵尸一旦被攻击就会放出一群僵尸鸡。

Once a satisfactory system of lubrication has been secured the hardness of the interacting elements becomes critical.
一旦获得了满意的润滑系统，相互作用的元件的硬度就成为紧要之事。

In other words, once we pass a certain critical point, the computers take over and there is complexity explosion.
换句话说，一旦我们过了某个临界点，电脑就会取得主宰性地位,出现一个“复杂性爆炸”。

something是什么意思：
pron. 某物；某事
n. 重要的人（或事物）；大致

It is not something ephemeral.
它不是朝生暮死的东西。

Know something of everything and everything of something
既要有一般常识,又要有专业知识

Know something of everything and everything of something.
通百艺而专一长。

happens是什么意思：
v. 偶然遇到；发生；出现；碰巧

These things will happen.
这种事总要发生的。

It happened far back in the past.
这事发生在久远的过去。

A string of accidents happened at that corner.
在那个转角发生了一连串的事故。

once是什么意思：
adv. 一次，一回；一度；曾经
conj. 一旦；一…就…
n. 一次；一回

Once is no custom
一次不成例

Do it for this once.
就干这一次吧。

Gordon is at once modest and clever.(at once …and…既..又)
戈登既聪明又谦逊