all at once
1.突然suddenly
He altered his face all at once.他突然变了脸。
I felt afraid all at once.我突然间感到害怕。
2.同时at the same time
You can't do everything all at once.你不能同时做每一件事。
How is it possible to feel love for two men all at once?怎么可能同时爱上两个男子呢?

at once
立刻immediately
I'm leaving for Rome at once.我马上就去罗马。
He offered to go at once.他主张立刻就去。

Once is enough for me.我一次就够了。
She's only done it the once so don't be too angry.她只做过这一次,所以不要太生气。

at once, directly, immediately, instantly, presently, right away
参见immediately条。
at once, soon
参见soon条。

［同义词］ adv.  at one time
［词源］＜中古英语ones(一)

once again
一而再,再来一次now again, as in the past
My mother asked me once again to take great care of myself.妈妈再三要求我多当心自己的身体。
He told me once again to be sure to put out the light before leaving.他曾再三叮嘱我在离开时一定要先关灯。

once and for all
一次了结地,一劳永逸地for the last time
He will explain it fully once and for all.他将把它充分地解释一次就此了结。
The teacher has warned the student once and for all.那位老师已断然警告那个学生。
I'm telling you once and for all, if you don't stop pestering me you'll be sorry.我这是最后一次警告你。如果你不停止纠缠我,你将来会后悔的。

once in a while
有时,偶尔sometimes, but not often
He is as busy as a bee once in a while.他有时也像蜜蜂一样忙碌。
In everyday life you may have a brush with other people once in a while.在日常生活中,你偶尔会与别人有点小摩擦。

once more
再一次now again as before
He's back home once more.他再一次回家了。

once or twice
几次several times; a few times
I've been there once or twice.那儿我去过几次了。
I made the same mistake once or twice.同一错误我犯了好几次。

once upon a time
从前some time ago; formerly
Once upon a time there was a giant with two heads.从前有个两头巨人。

I have been there once.我到过那儿一次。
I once went around the world.过去我周游过全球。

1.once用作连词时可引导时间状语从句和条件状语从句。用于前者时意为“一旦…就…”,所引导的从句多很短; 用于后者时意为“只要”,这种条件从句似带有时间条件的含意。
2.once引导的从句中有些部分可以省略。

Once you begin, you must continue.一旦开了头,你就必须继续下去。
Once you have taken the eｘamination, you'll be able to relaｘ.你一旦考过试,就可以轻松一下了。
Once you have promised you must do it.你既已答应,就应该去做。
Once inside her bedroom, she began to cry.一走进卧室,她就哭了。