"Please correct the sentence below." "Please let me explain to you some of the factors that negatively affected the sales for May" OR "Please let me explain to you why the sales for May were so low/poor/bad". Reasons explain things; reasons don't affect anything. So you don't say "reasons that have affected". Factors affect things.

Here is my version : "Please allow me to explain to you some of the factors that have negatively affected sales in May."

I second that. It's definitely better (easier) for you (Emily) if you post in your Notebook because then you can see where changes were made. :) It's actually not easier for us, though, because we have to highlight each change (that is, for folks who bother doing that). ;)

It is easier to correct your writing if you post it in your Notebook!