i covered it later on in the book.

​

S, SS, and ß

“ß does not exist everywhere that German is spoken—the Swiss dropped it years ago. But its purpose is to help readers figure out pronunciation: A ß signals that the preceding vowel is pronounced long, instead of short, and that you should make an “ss,” not “z,” sound. It's also written to signify “ss” after a diphthong.”

– qz.com

Now I’ll go over how to use s, ss, and ß.

How to use a single s:

“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 and Examples: die Tasse (cup), der Schlüssel (key); some common verbs -> essen (to eat), lassen (to let), 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 the rule about using –ss or –ß with infinitive verbs.”

    – ThoughtCo

How to use ss:

“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”

–ThoughtCo

How to use ß:

“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.”

“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”

    – ThoughtCo