Welcome, @Silke! Yes, this is what usually happens to the sound of the letter S:
When followed by a vowel in fast speech, it sounds like a Z (as in zero) and there’s a kind of “liaison” with the following word:
Desculpe, mas eu não entendo => Desculpe, ma[z]eu não entendo (I’m sorry, but I don’t understand)
When followed by a vowel in slow speech, the more each word is pronounced independently, the less apparent is the Z sound and “liaison”, so the S might also sound like SH (as in shoe) here.
Tens… um… livro? => Ten[sh]… um… livro? (Do you have a book?)
When followed by a consonant, it also sounds like a SH (as in shoe).
Falas português? => Fala[sh] português?
For beginning beginners like me, this is what inhibits learning, the inability to comprehend certain words and combinations due to fast speech. I speak very slowly, and probably always will, because that is the nature of the learning game! If I lived in Portugal and had to interact with people all the time in Portugues, then the problem would eventually minimize. but for those of us who cannot do that, we have a harder time with the lingua.
You’re both very welcome. Yes, the fast speech and the way it affects pronunciation are a big challenge. But if you learn this kind of patterns, it’s easier to slowly start figuring it out.
I just noticed this myself while going through the units here. I always thought the ‘S’ at the end of a word was always pronounced as ‘sh’ without regard for whether the starting letter of the next word is a consonant or vowel. I am also a little bit confused about when there should be a liaison with the vowel of the next word or not. For example, ‘Tens algum tempo livre?’. In your lessons, the ‘tens’ was pronounced with the s as just ‘s’. In your explanation above … ‘tens um livro?’ - the ‘s’ is pronounced with a ‘sh’. Can you explain a little bit more please?
You’re welcome, @beth.dizon The “liaison” tends to happens with any vowel in any word when speaking fast. Also with words starting with the letter H, because for us, it’s always mute (e.g., “Tens horas?” [What time is it?] sounds like “Ten[z] horas?”) It’s only when speaking so slow to the point of isolating each word that the “sh” sound comes out more. That’s why I stressed in my explanation that it only applies to slow speech. I should also add that I’m describing standard Portuguese pronunciation, but regional accents may not always stick to these patterns.