(1) If a word ends in a letter which cannot add -s and be pronounced. Such are box, cross, ditch, glass, lens, quartz, etc.
-Es added in certain cases.
If the word ends in a sound which cannot add -s, a new syllable is made; as, niche-niches, race-races, house-houses, prize-prizes, chaise-chaises, etc.
-Es is also added to a few words endi