Introduce plurals after ss, zz and x. To make hat into hats, you simply add an s. But to make the word kiss a plural, you need to add es. ES is added to words ending in ss, zz and x.
It is good to note that the sound of s changes depending on if it follows voiced or voiceless consonants. Voiced consonants create a vibration produced in the voice box. With voiceless consonants there is no vibration. The sound is produced in the mouth. After voiceless, it sounds like s. After voiced, it sounds like z. ES always sounds like z because the s follows a vowel and all vowels are voiced.
Below is a list of voiced and voiceless consonants.
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