Some examples:
For an English plural, the answer is never. Some words ending in the letter 's' are made plural by adding 'es', such as one bus, two buses or one class, two classes.
Two plural words ending in '-s':table > tableschair > chairs
Inions and iodins are the two longest words; some others are:dindodonIidininnionisnonodonsinsosodson
For P waves, the two descriptive words are Primary (or compressional) waves, while for S waves, the two words are Secondary (or shear) waves.
Some words that begin with S and end with W are:sallowSawscarecrowscowscrewseesawsewshowshrewsowspewswallow
Some words that start with S and end with B are:sahibscabscarabscrubshrubslabslobsnobsnubsobspareribspiderwebsquabstabstubsubsuburbsuccumbsuperbswab
Some small words that end with 's' are:isasushashisgasitsbuswasthis
Some words with a silent S:AisleAproposArkansasBourgeoisChamoisChassisDebrisHors d'oeuvresIllinoisIslandIslePrecisRendezvous
damp
Some plural words ending in "o" are photos, pianos, casinos, and zoos.
xerox
swimming and swam