S and Z sounds
The English S is a very tricky letter. In some words, it sounds like an S, but in other words, it sounds like a Z. It is very important to understand this. It's also important to understand that you cannot tell by the spelling of the word whether you should use an S sound or a Z sound.
To give you an idea of how confusing it can be, think about the words CLOSE (near) and CLOSE (the opposite of open):
The first CLOSE (near) is pronounced with an S sound.
The second CLOSE (the opposite of open) is pronounced with a Z sound.
Obviously, there is no difference in spelling, so how do you know which sound to use for these two words? The answer is, you just have to know the difference. Native-English speakers became familiar with these two words at a very early age, even before they learned to spell them. For non-native speakers it might be a little harder, but it is still possible. You just have to listen very closely whenever you hear a native-speaker use them.
The Z sound, which is really a voiced S sound, does not appear in Tagalog. Start with the S sound first, to get a feel for the mouth position, then, instead of blowing air, use the humming technique to create the Z sound.
Most Filipinos can make the sound, but the problem for many is simply knowing when to use it. Some Filipinos, who are aware of the S/Z issue, swing to the extreme, pronouncing all S's as Z's. This is also not correct. Once you have mastered the sound, concentrate on using it in the appropriate places.
One last point: Sometimes a word has more than one S, like SALSA and SEASON. Be very careful with these words, and listen to each S separately. In SALSA, both S's are pronounced as S. but in SEASON, the first S is pronounced as S while the second is pronounced as Z.
Practice with the following words.
In these examples, the letter S is always pronounced as Z:
In these examples, the letter S is always pronounced as S: