all at once
1． all of a sudden；suddenly；unexpectedly突然；出乎预料
All at once they found themselves surrounded by the enemy．他们突然发现自己被敌人包围了。
All at once she decided she wasn't going．She didn't explain why．她突然决定不去了，她没有讲明理由。
All at once we heard a shot and the soldier fell to the ground．突然我们听到枪声，接着士兵就倒下去了。
2．at the same time；together同时；一起
When he heard the news， he was happy and sad all at once and didn't know what to do．听到这消息，他悲喜交集，不知所措。
She couldn't tell them more about the accident．Everything just seemed to have happened all at once．她不可能向他们讲述更多的事情，一切都似乎是同时发生的。
3．without delay马上；即刻
We just can't decide such an important thing all at once．我们只是不能马上决定这样一件重要的事情。