Rule 5: Suffix Begins with a Vowel
When the suffix begins with a vowel, double the final consonant only if (1) the word has only one syllable or is accented on the last syllable and (2) the word ends in a single vowel followed by a single consonant.
Examples:
tan + ing = tanning (one syllable word)
regret + ing = regretting (The accent is on the last syllable; the word ends in a single vowel followed by a single consonant.)
cancel + ed = canceled (The accent is not on the last syllable.)
prefer + ed = preferred