In English, an -S is added to words to make them plural, possessive, or to change the verb tense.  Even though the letter is always an -S we pronounce this sound differently depending on the sounds the precede it.

If the word ends in a sibilant sound that ‘hisses’ like s, sh. z, ch, j, zh; then pronounce the -S as IZ.

If the word ends in a voiceless sound t, k, f, th, p; then pronounce the -S as an S sound.

If the word ends in a voiced sound: m, n, l, b, d, g, v, th, r; then pronounce the -S as a Z sound.

For example:

BUSES is pronounced bus-iz

CATS is pronounced cat-s

CABS is pronounced cab-z

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