The final "s" has 3 different sounds: s, z, or iz. The correct pronunciation is based on the final sound of the word before the "s" or "es" is added. This applies to "s" or "es" added to nouns to make them plural, and "s" at the end of verbs in the present simple 3rd person singular. (He walks to work every day.) You've studied this, now it's time to see if you learned it. There is also a short list of commonly used words in which the final "s" is pronounced as "z" GOOD LUCK!