keyword = {
      "shortkey":"longer sentence",
      "gm":"goodmorning",
      "etc":"etcetera"
}

with open('file.txt', 'r') as file:
   with open('newfile.txt', 'a') as newfile:
      lines = file.readlines()
      for line in lines:
         if 'shortkey="' in line:
            x = line.split('"')[1].split()
            if x == keyword.keys():
               for x, replacement in keyword.items():
                 newfile.write(replacement)