Which is not a rule for changing the spelling when adding suffixes beginning with vowels?

The assertion that double the final vowel if it is a short vowel sound is not a rule for changing spelling when adding suffixes that begin with vowels is accurate. In English spelling conventions, it is the final consonant that typically gets doubled after a short vowel sound in single-syllable words, not the vowel itself.

This means that, for example, in the word "run," when adding a suffix such as "-ing," you double the final consonant to form "running," but you do not double the final vowel, because that is not a standard spelling practice. The rules focus primarily on the consonants adjacent to the vowels in what is known as CVC (consonant-vowel-consonant) patterns.

The other choices detail actual established spelling rules: for one-syllable words that contain a single short vowel sound, the final consonant is often doubled; and words that end with a silent 'e' typically drop the 'e' when a suffix beginning with a vowel is added. Moreover, the rule about two-syllable words specifies that if the stress is on the last syllable, the final consonant should be doubled, elaborating on common exceptions rather than proposing incorrect methods of handling vowel endings.