input: 
  def subtitles_client_usage(command: str, releases: str, engine: str, language: str) -> str:
      """This function simulates the usage of the Subtitles Client command-line interface. It takes a command, one or more release paths, an engine for the subtitle site, and a language as inputs. It returns the output that the command-line would produce based on the input arguments.
      >>> subtitles_client_usage('download', 'movie.mkv', 'opensubtitles', 'en')
      "[     opensubtitles] Trying To Download Subtitles For: 'movie.mkv'\n[             \t] Download Success: (./movie.en.srt).\n"
      >>> subtitles_client_usage('exist', 'tvshow.mp4', 'subscene', 'fr')
      "[         subscene] 'tvshow.mp4' - Does Not Exist.\n"
      >>> subtitles_client_usage('test', '', '', '')
      "Testing engine...\n"
      """
  
output:     if command == 'download':
          return f"[     {engine}] Trying To Download Subtitles For: '{releases}'\n[             \t] Download Success: (./{releases}.{language}.srt).\n"
      elif command == 'exist':
          return f"[         {engine}] '{releases}' - Does Not Exist.\n"
      elif command == 'test':
          return "Testing engine...\n"
      else:
          return "Unknown command.\n"