My students often ask during their German lessons if they can just replace ß with double ss. The answer is NEIN, unless you are learning Swiss German where the letter ß does not exist and is indeed replaced with a double ss.
How do you know when it is ß and not double ss? Quite simple: when you have a short vowel before a sharp s-sound then there is a double s. If the vowel is long or there’s a diphthong then there is an ß.
dass, der Fluss, lassen, fassen
das Maß, die Straße, heißen, weiß
Want to learn more about ß? Check out our previous blog article: http://www.languagecoach.co.uk/double-s-or-s-you-may-wonder/