Here's why:
* Short vowels are typically represented by a single vowel letter and have a short, quick sound.
* Long vowels are often represented by a single vowel letter followed by a silent "e" at the end of the word, or by a vowel digraph (two vowels together).
Since "ran" has only one vowel letter ("a") and doesn't have a silent "e" or a vowel digraph, the "a" sound is short.