At once means immediately, and also all together, at the same time. In a story, the first meaning sounds neutral (I knew at once that something was wrong), but in a request it sounds strict, almost like an order: Come here at once! Right away is softer. The second meaning is about several tasks or people at the same moment: I can't do three things at once.
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do something at once (= immediately); everyone / two things + verb + at once (= at the same time); all at once (= suddenly)
- When the fire alarm went off, everyone left the building at once.
- Don't try to learn fifty new words at once; ten a day is plenty.
- My old laptop freezes if I open too many apps at once.
What changed
First versionI'll send you the file at once after the meeting.
RevisionI'll send you the file right after the meeting.
Since at once means immediately, at once after can look logical. But at once doesn't go before after or before. Use right after or straight after instead, or immediately after in formal writing.
The opposite of the second meaning is at a time with a number: Please come in one at a time. At once means all together; one at a time means taking turns. There's also all at once, meaning suddenly: All at once, the lights went out.
Your turn
Please don't all speak ___! I can't hear what anyone is saying.
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at once — Several people are interrupting the speaker, so the meaning needed is “all together”: at once. At a time is used only with a number and means the opposite: one at a time, one person after another.
Describe a time you got an urgent email at work or university. Say who it was from and what you did straight away, using at once.
My manager emailed me about a mistake in the invoice, so I fixed it at once.
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