一気に means "in one go": an action runs from start to finish without a break, or a change happens all at once instead of bit by bit. It is an adverb and goes before the verb.
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I read this book in one go.
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I finished my homework in one go.
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He drank the beer down in one gulp.
Verbs that say a job got finished sit naturally with it: , , , .
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Let's get the rest of the work done in one go.
A change all at once
With a verb of change, 一気に says the change came in one big jump.
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The temperature dropped all at once.
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The atmosphere changed all at once.
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Thanks to the new product, sales shot up all at once.
一気に, すぐに or 急に
says how soon something starts: "right away". 一気に says how it runs once it has started: without stopping.
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I did my homework right away.
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I did my homework in one go.
says a change came without warning. 一気に says it was large and came in one go, whether or not anyone saw it coming.
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It suddenly started to rain.
Lingaku