's is only used to mean 'van' with personal names or family words that end in a, i, o, u, y, e (when pronounced as a long 'ee'). For example: Renee's boek Opa's bril It's not used when a name ends in a short 'e' or in a consonant. Instead, it becomes only an s without the apostrophe: Mariannes fiets Douwes school Martijns schoenen etc. When referring to other nouns, you wouldn't say "de auto's ruit". Rather, you'd say 'de ruit van de auto'. So no genetive with s or 's.

's can be several things: 1. Possessive (John's shirt = Jans shirt; the car's windshield = de auto's ruit) 2. Contraction (It's three o'clock = het is drie uur; He's seen me = hij heeft me gezien) 3. It can NOT denote a shortened word like "eens" in Dutch. A classic mistake is the phrase "Spring is in the air." Many Dutch speakers might hear "Spring 's in de lucht." rather than "De lente zit in de lucht."