Answer:
The answer is changing the vowel Y to an I.
Explanation:
There is a general rule which says that if the word ends with a vowel Y preceded by a consonant, it changes to an I when added a suffix. So if we take the root of the word beautiful which is beauty, we see that it ends with a vowel y with the consonant t before it, so when adding a suffix -ful the vowel changes. E.g. lady - ladies, pity - pitiful. However, you should bear in mind that this rule just like any other has exceptions.
