Rule #1 If a word ends in a sound that is unvoiced (such as P,T,K,F), you add an unvoiced /S/
(the ‘p’ in cup is unvoiced, so you just add an unvoiced ‘s’)
(the ‘t’ in cat is unvoiced, so just add an unvoiced ‘s’)
Rule #2 If a word ends in any of these sounds: ‘s,z,sh,ch,or dg (j)’ when adding an ‘S’ ending, add… IZZZZZZ
Rule #3 If a word ends in a vowel sound (like the word Tree) or a voiced consonant (like the word Game), then when you add an ‘S’, continue the voicing throughout the entire word, and it should become a voiced ZZZZ.
2 Treezzzz (correctly spelled Trees)