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The class will be over in 10 minutes.

The class will be over after 10 minutes.

I know the first is correct but why?

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How do you "know the first is correct"? – Kris Jan 15 '13 at 13:22
It's correct because native speakers don't say the second sentence. – pazzo Jun 7 '15 at 16:06

In OP's examples, "in" and "after" both specify a future time relative to the present moment. There's no grammatical rule saying either preposition is correct or wrong - it's just idiomatic preference that most people would use "in".

Given that "in" is a somewhat "metaphoric" usage here, I suspect there's a tendency to only use it in simple constructions where it's relative to the present moment.

When speaking of some situation in the past, there is no "present moment" - so we need to explicitly state the time/event that we're counting our 10 minutes from. In such contexts, we're more likely to use after (or within, following, etc.) because the whole situation is more complex, so we choose our words more carefully. "I went to the bar, but I left after 10 minutes".

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The owner told me that I had to leave in 10 minutes. Here in sounds better than after. – Noah Jul 11 '12 at 5:45
And what about the class will end after 10 minutes of questions? – TimLymington Jul 11 '12 at 9:43
@TimLymington: In that case, the base "temporal reference" is the time at which the question period started. Maybe didn't express myself well in the answer text, but what I meant was that "in" usually implies the reference is "now, the present moment". For all other contexts, "after" is more suitable. – FumbleFingers Jul 11 '12 at 20:32
@Noah: Per with my comment to Tim, in your example, "in" means at the (then) "present moment" when the owner was speaking. – FumbleFingers Jul 11 '12 at 20:33
@Noah, I would read a difference between in and after in your sentence: If the owner told me that I had to leave in 10 minutes, he said that 10 minutes from now, I must depart. If he told me I had to leave after 10 minutes, then it took 10 minutes after I arrived for him to tell me to leave (immediately). – Hellion Jul 13 '12 at 13:32

They are both correct; although, if I wanted to be more precise, I would probably say it like this:

The class will end in 10 minutes.

Going back to your two examples: right now, class is in session, but in ten minutes, class will end, and after 10 minutes, class will have ended, (or, put another way, class will be over).

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Either one can work, however, because we don't usually give much thought to the relationships between time, changes of state, and times of state transitions – not in general conversation, at least. If it's now 12:50, and class runs until 1 o'clock, we don't generally make distinctions between 1:00 (when the bell rings) and 1:00:02 (when we're in the process of standing up) and 1:00:37 (when we are walking out the door). All we really pay attention to is, "Class is done at one!"

In a scientific paper describing some low-level process, we might need to pay more attention to our prepositions, and not say "after" when we mean "in", and vice-versa. For the end of class, however, either preposition suffices.

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To me, 'After ten minutes' suggests certainly not before ten minutes have passed, and might be quite a bit later. 'In ten minutes' is fairly precise but might mean a little less than ten minutes.

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A (The) significant point that everyone seems to have missed. – Kris Jan 15 '13 at 13:21

In this context, I believe 'in' and 'after' are prepositions because there is a time constraint. I think 'in' is used for near future and 'after' is for far future.

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um.. I don't see the difference... – deutschZuid Jul 11 '12 at 3:40
I don't think so. "I left after 10 minutes" is probably more likely than "I left in 10 minutes" in most contexts. – FumbleFingers Jul 11 '12 at 3:46
ah yes.. I didn't think about that while giving my 2 cents :-).. thanks :-) – thandasoru Jul 11 '12 at 5:29

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