If, when you say the words ‘keys’ and ‘kiss’, they sound the same, then there is a pronunciation mistake.
It is another very common pronunciation mistake of Spanish speakers, due to the absence of the sound ‘z’ in the Spanish language.
Basically, you can train the pronuciation of this sound, trying to immitate a mosquito sound. It has the same tongue-to-teeth position as when you pronounce ‘s’. Only that you have to add some voice. Try it now;)
Now, there are some rules that will help you to distinguish the situations when to pronounce ‘s’ and when ‘z’.
Here is the rule to help you:
When the letter ‘s’ is after an unvoiced or quiet consonant, it is
pronounced as a /s/. e.g., hats, tops, works, laughs, what’s, moths.
When the letter ‘s’ is after a vowel, another ‘s’, or a voiced consonant,
it is pronounced as a /z/ sound.
e.g., logs, tubes, beds, moves, clothes, was, becomes, he’s, passes.
Pronounce the ‘es’ as /əz/after the consonants /s /z/ ‘ch’ ‘sh’ ‘j’ ‘zh’ ( as in
vision)- (in ‘es’ /əz/, the schwa sound /ə/ is said like a very short ‘u’ sound)
e.g., passes, beaches, washes, packages, noses.
The other thing to remember is to pay attention when listening to native English speakers and the audio trainer in my program, and remember how they use it.
It’s time to practise!