Difference between "by", "for" and "per"
Can someone explain me when I have to use each one of those words? When do I have to use "by", when "for" and when "per"? Thanks a lot.

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Per can also mean "according to" or "in accordance with."  For example: 
-I completed the project per your email.
-He assembled the furniture per the assembly instructions.

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'Per' is used to describe ratios. For example, km/h is pronounced "kilometres per hour". m/s is "metres per second". I think that's pretty self-explanatory.

'By' and 'for' can mean lots of things and they both are in a lot of phrasal verbs. Here is something I typed up about their general meanings:
http://www.italki.com/question/204803
Be careful of their exceptions.