Can anyone please explain the difference between these two sentences
1) I've not slept in three days. 2) I've not slept for three days.
I hear people using the both sentences. I'm just curious if there's a difference between them or not.
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There are some contexts, including this one, where both prepositions fit. They mean the same thing. For the past three days (and presumably nights) the person has not slept.
But there are other contexts in which they don't mean the same thing.
He was locked up for three days
means that for a period of three consecutive days at some point in the past, he was locked up. In this case, the use of in is not possible.
He will be locked up in three days
means that in three days from now, he will be locked up for an uncertain period. Here for does not fit. It does not mean that he will be locked up for three days.
So, the choice of in or for depends on the context. Sometimes you can use either; sometimes you can't.