One very common struggle for second language learners is understanding the voiced and unvoiced sounds, particularly S and Z. S and Z use the exact same tongue and mouth shape; the difference is in the voicing.
The S sound is produced without the voice, but the Z sound requires the voice.
To feel the difference, put your hand on your throat and try saying S-s-s-s. You should not feel anything.
Now try saying Z-z-z-z. You should feel a vibration in yourthroat. You should also be able to feel the difference if you put yourhands over your ears or on your cheeks. You should feel them vibratewhen you say Z, but not when you say S.
I have created a video which demonstrates the difference between the two sounds, and also gives you some practice words to try. You can watch it below and practice along with me.