My students often ask during their German lessons if they can replace ß with double ss. The answer is NEIN unless you are learning Swiss German where the letter ß does not exist and is 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, 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? Enquiries about private German lessons are welcome.