Pronouncing the suffixes (also known as word endings) -s and -ed are usually a big problem for learners of English, particularly for Vietnamese (and probably other Asians). Using and pronouncing them don't make sense for Vietnamese learners since their language doesn't use such features. Here you have a short simple guide on pronunciation of the -s and -ed suffixes.
The -s ending of verbs and nouns can be pronounced in three different ways: -s, -z, and -iz.
The termination –s/-es is pronounced /s/ after UNVOICED consonants except /s/, /ʃ/, and /ʧ/.
Example: hopes, picks, starts, books.
The termination –s/-es is pronounced /z/ after VOICED consonants except /z/, /ʒ/, and /ʤ/
Example: begins, studies, goes, needs.
The termination –s/-es is pronounced /iz/ (or /əz/ in some cases) after /s/, /z/, /ʃ/, /ʒ/, /ʧ/ and /ʤ/.
Example: boxes, watches, kisses, washes.
The -ed ending of verbs in past tense can be pronounced in three different ways: -t, -d, and -id.