> ダイエットは３日で止めて仕舞った。

My loose translation would be

> I already gave up on the diet after 3 days.

And a more precise one

> As for the diet, I regrettably stopped it after 3 days.

From my understanding, で is used to say "after". But で usually marks the
method by which an action occurs.

So, is my translation "after 3 days" correct? Can you name a more precise
translation for the example? And what is the semantic of で in respect to this
example?
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2021-07-14T13:44:04.017
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question
meaning particle-で
Using the で particle for time limits: ダイエットは３日で止めて仕舞った
Your translation sounds nice and correct. 仕舞った（しまった） sounds a regrettable
nuance so the second one is also done well.

* * *

Other than marking the method, で may appear next to a duration when you do
something in the (limited) duration.

> A: 夏休{なつやす}みの宿題{しゅくだい}を[3日]{みっか}で[終]{お}わらせたよ。」  
>  (I finished the homework of the summer holidays in 3 days.)  
>  B: [3日]{みっか}で！？  
>  (Only 3 days!?)

* * *

> A: [手伝]{てつだ}ってくれない？[5分]{ごふん}でいいから  
>  （Could you help me? It'll take only 5 minutes.）  
>  B: [忙]{いそが}しいけど[5分]{ごふん}ならいいよ。  
>  (I'm busy now but, okay, only 5 minutes.)
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answer