Vowel Suffix ES is very similar to Suffix S. 

Suffix -ES  can make the base word mean more than one, or plural, as in “glass” to “glasses”  or it can change a word to show “the action of another,” as in, “I pass.” to “She passes.”

Suffix -ES is used instead of Suffix -S when the base word ends with S, Z, X, SH, CH, and TCH.

When used, Suffix -ES creates its own unaccented syllable and has a schwa sound. 

Suffix -ES, Suffix, -ES, Suffix -ES, passes, glasses