1.just the once只有一次
2.all at once
(suddenly)突然；忽然
All at once there was someone knocking on the door.忽然有人敲门。
(simultaneously)同时；一起
3.at once
(immediately)立即；马上
I knew at once that something was wrong.我立刻意识到情况不妙。
(simultaneously)同时
They all arrived at once.他们都是同时到达的。
Everybody is talking at once.所有人都同时在说话。
<文>(both)既…(又…)
It was at once a question and a command.这既是询问, 也是命令。
4.for once
(exceptionally)仅此一次；破例一次
She was able, for once, to relax.只有这一次, 她能够放松一下。