The study of rules and uses for vowels falls under the study of phonics. Whether a sound is pronounced as a long or short vowel is based on a set of vowel rules pertaining to spelling patterns in the English language.
There are several long vowel rules to follow. First, if a word has two vowels, the first vowel is usually articulated while the second vowel is silent. For example, in the words "rake," "meal" or "road," the first vowel makes the sound while the second is silent. Another long vowel rule states that if a word has one vowel and it comes at the end of the word, that vowel usually has a long vowel sound; for example, "he," "she" or "why." A final long vowel rule states that the vowels "i" and "o" usually have the long vowel sound when followed by two or more consonants. Examples of this rule are the words "light," "kind" and "volt."
The short vowel rule states that if a word has one vowel, either at the beginning of the sentence or between two constants, it usually has a short vowel sound. Examples of this rule include the words "ant," "west" and "swim."
Another common vowel rule states a word should be prefaced by the word "a" or "an" based on whether or not the word begins with a vowel. For example, "a ball" versus "an umbrella." The word "ball" starts with a consonant, so it requires the "a" article. The word "umbrella" begins with a vowel, so it requires the use of "an."