Here's why:
* Short vowel sounds are typically found in words with one syllable, where the vowel is followed by a consonant.
* Long vowel sounds can be found in words with one or more syllables, and they often have a vowel followed by a vowel or a silent "e" at the end.
In "happen", the "a" in the first syllable is followed by a consonant, but it has a long vowel sound, as in "hay".