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 when double ss? Quite simple: when there is 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 one of our previous blog articles: https://www.languagecoach.co.uk/double-s-or-s-you-may-wonder/