when you call a function you put the variable name not the variable name and type. If you put the variable name and type then you are by definition defining a new variable when you really want to use the old one. Also you spelled sentence wrong on line 18 it should be spelled how it is on line 16.
Also line 25 looks like an infinite loop to me. Maybe you want it to loop while argc is greater than 0?
On line 18, you want to supply "sentance" to your function, like so:
Also, your while(--argc) code is fine, if a bit weird. For anyone who doesn't understand it, it is looping through, subtracting one each time. The "while" loop tests if it is true (non-zero) or false (zero), and the loop ends when argc reaches 0, allowing the loop to finish.