(1) If a word ends in a sound that hisses (/ch/, /s/, /sh/, /x/, /z/), add -es.
(2) If the last letter of a word changes in plural, add -es. There are two situations:
(a) If the word ends in a consonant + y, change y to i and add -es.
(b) In most cases, if the word ends in f, change f to v and add -es.