Future perfect with 'by' and 'on' What's the difference between the following 
1.You will have received it by Monday.
2.On Monday, you will have already received it.
What is the difference between the two? To me both are different ways of telling the same thing. 
Thank you

There are many differences between the sentences. Let's go through a few.

You will have received it by Monday.

The thing that's tricky about this sentence is that it has the future perfect tense (i.e. suggesting actions that will be done, but haven't been done yet, but should (ideally) be completed within a certain time frame). 
It means that someone promised you you'd receive it by Monday, and he's already thinking about what happens after that point. The 'by' here makes it tricky too by implying "at least, at no later than, not after," and as a result, means that you can start working on something else or be expectant of other conditions that arise due to its arrival.

On Monday, you will have already received it.

This implies that when Monday happens (it hasn't happened yet, but it will), it will be there. It specifies the range of time which could be long down to a small one, suggesting that someone has done their work ahead of time, and therefore you are able to receive it by the time Monday happens.
If you want to read more on this subject, I suggest the following two websites:


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*ESLCafe (Grammar lessons: Prepositions #2) 

*EpressoEnglish (Prepositions of Time)