The form Subscript[x, n] is called the inputform. The entry Subscript[x, n] will evaluate (when you evaluate the cell or notebook) into an output x
n. If you wish to have your textual entry immediately converted into x-subscripted-n then select the words Subscript[x, n] and press <ctrl + shift + enter>. Hope this is what you are looking for.
PS: My apologies, I did not notice earlier that the query was about Wolfram Alpha. The above applies only for Mathematica I would think.
