The correct rule when adding a suffix beginning with a vowel to the word "desire" is to drop the final e. For example, when suffix -able is added, the word + suffix produces the new word "desirable". This rule is also applicable to similar words such as "admire" and "advise".
This is something we can't really answer, because we don't know your experience learning spelling words. There isn't one specific answer for this problem, it's determined by the person answering it.
