Difference between 'You' and 'you' forms

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Nov 1, 2012 16:35 grammar idiom You you
I was meeting with one problem in English grammar a couple of times.
More often I have this question at the work when I had been answering our customers' letters.

Should I use 'You' or 'you' forms in these kinds of letters?
Or is n't there 'You' form in English grammar at all?
How can I show my respect to our customers in letters or in person?

I have been losing in a such simple question: between 'you' and 'You'.
Or as saying in Russian - I have been got lost in three poplars
:)