Q1: why do I need a equal sign after + ? why not just "sum + i
As curiouskiwi does a great job pointing out, sum += i is really the same as sum = sum + i. (just a little syntactic sugar meaning it's easier to type and computer scientists are lazy by nature).
So really you are asking what is the difference between sum = sum + i and sum + i. And the difference is the assignment operator "=". The first option tells the processor to add i to sum and the STORE IT in the variable sum (well the address that sum points to technically). Whereas the second option tells the processor to add i to sum, but then doesn't say anything else.
It's kind of the equivalent of opening up a document, writing your essay and then closing it without saving.
Hope that helps.