2. The pronunciation of 's'

Whether you add -s to make the third person singular present or you put a noun in the plural form or you add -'s to make the possessive, the -s you add should be heard when you say it orally.

The pronunciation depends on the last sound of the verb or noun as the letter -s can be pronounced either /s/ or /z/ or  /iz/


Rule 1

If a word ends with the sounds /s/, /z/, /ʒ/, /ʃ/ /tʃ/ or /dʒ/ the final -s is pronounced /iz/.

Rule 2

If a word ends with any other voiceless consonant sound (/p/, /t/, /k/, /f/, or /θ/), the final –s is pronounced /s/.

Rule 3

If a word ends with any other voiced consonant sound (/b/, /d/, /g/, /v/, /d/, /l/, /r/, /m/, /n/, or /ŋ/), or a vowel sound, the final –s is pronounced /z/.


Source: http://epronunciation.com/pronunciation-rules/pronunciation-words-ending-s-ed.html


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