Use the /s/ sound when the word ends in F, K, P, T or unvoiced th. In these words, before adding "s", the voice stops. Cats Likes Laughs Tops Cops Books Months
Use /z/ when the word ends in a vowel sound, or in a sound that is not F, K, P, T, or unvoiced th. In these words, before adding "s", the voice is active at the end. Days Trees Plays Fleas Bees Peas Runs Eyes Tables He's She's
Use /ez/ when the word, before adding "s", ends in /sh/ /s/ /ch/ sounds. Pieces Places Matches Watches Washes Kisses Fishes Punches