The limits should be underscripts and overscripts of in normal input, and subscripts and superscripts when embedded in other text.
Sum uses the standard Mathematica iteration specification.
The iteration variable i is treated as local, effectively using Block.
If the range of a sum is finite, is typically assigned a sequence of values, with being evaluated for each one.
In multiple sums, the range of the outermost variable is given first.
The limits of summation need not be numbers. They can be Infinity or symbolic expressions.
If a sum cannot be carried out explicitly by adding up a finite number of terms, Sum will attempt to find a symbolic result. In this case, is first evaluated symbolically.
Sum can do essentially all sums that are given in standard books of tables.