z c s and zh ch sh
A lot of foreign people could not pronounce zh ch sh correctly.
when we practise pronouncing z c s and zh ch sh, normally we add i to that, eg, zi ci si and zhi chi shi. In wikipedia, they are explained as following.
| zh | [ʈʂ] | ch with no aspiration (take the sound halfway between joke and church and curl it upwards); very similar to merger in American English, but not voiced |
| ch | [ʈʂʰ] | as in chin, but with the tongue curled upwards; very similar to nurture or tree in American English, but strongly aspirated |
| sh | [ʂ] | as in shinbone, but with the tongue curled upwards; very similar to undershirt in American English |
| r | [ʐ] or [ɻ] | similar to the English r in rank, but with the lips spread and with the tongue curled upwards |
| z | [ts] | unaspirated c (halfway between beds and bets) (more common example is suds) |
| c | [tsʰ] | like ts, aspirated (more common example is bats) |
Pronunciation of initials (From Pinyin wikipedia)
From my experience, when you pronounce zi ci si, the front part of your teeth bite each other. When you pronounce zhi chi shi, the two sides of your teeth bite each other.