at once

immediately, instantly, presently, directly, shortly, soon, at once, right away

这些副词或副词词组均有“立刻、马上”之意。
immediately: 指做完一件事后，立即就做另一件事，中间一般没有明显的时间间隔。
instantly: 指恰恰就在此刻，一秒钟也没耽误。
presently: 指不久、即将。
directly: 和immediately同义，指毫无迟疑。
shortly: 和soon同义，指动作发生或完成的速度之快。
soon: 常用词，指行动快，完成速度快。
at once: 口语中常用，语气强烈，指时间紧迫，一刻也不能耽误。
right away: 主要用于美国英语，语气稍弱于at once，强调动作迅速。

副词 at once:

1. without delay or hesitation; with no time intervening
   同义词：immediately, instantly, straightaway, straight off, directly, now, right away, forthwith, in real time, like a shot
2. simultaneously
   同义词：at a time, at one time

1. The man was so genial, so I was drawn to him at once.
   这人这么和蔼，我立刻为他着迷。
2. We agreed to leave at once.
   我们同意立即离开。
3. Since she is seriously ill, we have to send for a doctor at once.
   既然她病得厉害，我们要马上去请医生。
4. We determined to go to the railway station at once.
   我们决定立刻去火车站。
5. It is necessary that he be sent there at once.
   有必要马上派他到那里去。
6. When I gave him the chance, he grabbed it at once.
   我给他这一机会，他立刻抓住不放。
7. If you see anything suspicious, inform the police at once.
   你要是看到什么可疑的东西就马上通知警察。
8. He stopped playing the piano at once.
   他立刻停止了弹钢琴。