general rule : If your word ends with an sh and a vowel, add "es". If your word ends with an f, change f to v and add "es". if your word ends with a y, and there is a consonant before it, change y to i and add "es". and lastly, if you word ends with a y and there is a vowel before it, just add "s". Take note that there are a few words that are exempted from these rules.