here are three major points that beginner ESL/ELL students should understand about fricative sounds:
To produce fricatives, air travels smoothly through a small, constricted opening in the vocal tract
Fricatives are capable of being formed continuously, with no complete blockage of the vocal tract
Except for the h sound, fricatives occur in voiced/unvoiced pairs
There is one subtle, additional aspect of fricative sounds:
The duration of a vowel sound before a voiced fricative is greater than the duration of a vowel sound before an unvoiced fricative