Question

why do you pronounce the j in jefe and juan differently?
Is there some kind of rule for the letter j?

Answer 1

Answer: You don't.

Explanation: I'm a native Spanish speaker and as far as I know, there is no rule for the letter J. Both jefe (heh-fae) and Juan (who-awn) are both pronounced with the same H sounding J in Spanish.

Answer 2

Answer:

There is no difference in the sound of the letter j when you say ja, je, ji, jo, ju

Explanation: