Need help understanding function in BSL (DrRacket)

Question

Language BSL - DrRacket (racket variant)

Problem: I need to understand what the function below does exactly. Specifically the substring bit, I don't understand how it determines whether a string has an "?" at the end in choosing whether to append "?" to the consumed string.

(define (ensure-question str)
  (if (string=? (substring str (- (string-length str) 1)) "?")
      str
      (string-append str "?")))

Answer 1

(substring s position) function returns substring of string s from certain position. For example : (substring "Apple" 1) returns "pple" since we are taking subscript from the first character.

(substring "Apple" ( - (string-length "Apple") 1))

will return "e".

We are taking subscript including only the last character (- (string-length str) 1)) and checking whether it equals to "?" by using function string=?.