1) “立刻”、“马上”，修饰过去、现在和将来的一次瞬间动作。

• Come here at once.你马上过来。
• The audience at once greeted him warmly.观众立刻热烈欢迎他。
• He started at once to compose a reply to Anna.他立刻写信回复安娜。

2) “在同一时间内”，修饰过去、现在或将来的多件同时实行的瞬间动作。

• You can't be doing two things at once.两件事不能同时做。
• No bank could ever pay off its creditors if they all demanded their money at once.任何银行，如果债权人全都同时来索债，都是无法支付的。

3) 表示“同时”具有多个延续的状态。

• The book is at once funny and instructive.这本书既有趣又有教育意义。
• From childhood, he was evidently at once rebellious and precocious.他从小就显得既调皮又早熟。

at once 前面可以加 all，也同样有上述三个意义。但是用于第一个意义（“马上”、“立刻”）时，all at once 多出现在句首。

• All at once there was someone knocking on he door.立刻有人敲门。
• All at once, his serious expression softened into a grin.他那扳着的脸容一下子松弛下来，露出笑容。

用于第二和第三个意义（“几件事情同时发生”或“几个状态同时存在”）时，all at once 多出现在句末。

• She wanted to laugh and weep all at once.她真想同时又笑又哭。
• She is a teacher, housewife, and mother all at once.她同时又是老师，又是家庭主妇，又是母亲。