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It seems they can be used interchangeably but I am not sure. Can somebody explain the difference of use between these two key words? Where do they become different?

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You certainly can't use them interchangeably in all contexts - in can be used in many places where within is totally wrong. If OP is unsure, I suggest sticking to in, which would rarely if ever be unacceptable. – FumbleFingers Aug 31 '11 at 17:32
up vote 16 down vote accepted

Within is closer to inside - it mainly refers to something like a building where you are physically in (or inside).

In can also mean a state or an organisation, so you can be "in trouble", or "in school" but not "within trouble".

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+1 for neat answer. Thanks. – Tarik Aug 31 '11 at 17:34
The other answer makes a good point too: You can't always substitute in for within (or inside). – Bradd Szonye Apr 20 '13 at 11:38

where ever there is a certainty we use in and in uncertainty we use within.

I will come in next week. - Use IN-
I will come within this week - Use within

Also, if some timeframe is allocated ie : do the work in ten minutes

Then if the work is completed in 10 min then use in and if completed before 10 min then within.

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I will come in next week does not mean (what I surmise you are trying for) "I will come in the next week". "I will come within the week" means the latter, but the way to express your last point is "I will come in a week's time". – TimLymington Apr 20 '13 at 11:43

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