Start from this link and read through "next-page" till you get to long vowels, maybe a bit after.
A quick explanation: Horen is divided into syllables as Ho-ren. In Dutch, a vowel at the end of a syllable is always a doubled vowel, so you don't need to write it doubled. (ok, maybe a few words that are exceptions, but don't worry about that)
Paarden is divided as Paar-den. The a is pronounced double, but since it's in the middle of the syllable, you don't know that it's doubled unless you also spell it doubled.