When to Use s, ss or ß

I must admit it was hard for me let go of certain old spellings when the new German spelling reform first appeared in 1996. In particular I missed and had to get used to the changes of words with the sharp s (ß), such as daß -> dass, bißchen -> bisschen etc. For those of you who learned German after the spelling reforms - German spelling rules have been simplified! However, many German teachers would argue not enough.

For instance, it is still difficult for beginner students to sort out when to use s, ss or ß in a German word. (In Switzerland, there is no such confusion, since the eszett (ß) has been eliminated from Swiss-German for several decades.)
The following is a run-down of when to use s, ss and the infamous ß. But of course, beware of exceptions:

  1. A single –s is used:

    • At the beginning of words.
      der Saal (hall, room), die Süßigkeit (candy, sweet), das Spielzimmer (playroom)
    • Mostly in nouns, adjectives, adverbs and a few verbs when preceded and followed by a vowel. lesen (to read), reisen (to travel), die Ameise (ant), gesäubert (cleaned)

      Exception Examples: die Tasse (cup), der Schlüssel (key); some common verbs -> essen (to eat), lassen , pressen (to press), messen (to measure)
    • After consonant -l, -m, -n, and -r, when followed by a vowel.
      die Linse (lentil), der Pilz (mushroom), rülpsen (to belch)
    • Always before the letter –p.
      die Knospe (a bud), lispeln (to lisp), die Wespe (wasp), das Gespenst (ghost)
    • Usually before the letter –t.
      der Ast (branch), der Mist (dung), kosten (to cost), meistens (mostly)

      Exception Examples: Verb participles whose infinitive form have a sharp -s. See rule #4.
  1. A double ss is usually written only after a short vowel sound.
    der Fluss (river), der Kuss (der Kiss), das Schloss (castle), das Ross (steed)

    Exception Examples:
    bis, bist, was, der Bus
    Words ending in –ismus: der Realismus
    Words ending in –nis: das Geheimnis (secret)
    Words ending in –us: der Kaktus
  2. An ß is used after a long vowel or dipthong.
    der Fuß (foot), fließen (to flow), die Straße (street), beißen (to bite)

    Exception Examples: das Haus, der Reis (rice), aus.
  1. Infinitive Verbs with –ss or –ß:
    When these verbs are conjugated, then these verb forms will also be written with either –ss or –ß, though not necessarily with the same sharp –s sound in the infinitive form.

    reißen (to rip) -> er riss; lassen -> sie ließen; küssen -> sie küsste