I am learning German, but cannot write grandfather in Deutsch as the English Keyboard does not allow it. So it is continually wrong. So frustrating. Also if my spelling is incorrect it throws me out. Even though I was learning to speak and identify the words. I was liking the App but it is quickly becoming a time waster.
Are you using an iphone or android? If yes, then you just hold the letter down and many options are available for each letter that you can scroll over and choose. The letter you are looking for is available with the 's'. You can also get keyboard layouts that will work on your keyboard for any language. http://duolingo.wikia.com/wiki/Guide_to_keyboard_layouts_and_input_methods