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

After asking my friend about some ways they learn spanish, they told me that that use study groups to to improve there spanish skills also they constantly study and memorize the vocab and grammar. i know that the subjunctive verb is all about the mood and how the speaker feels. so anytime i have an assignment that involves subjunctive verbs i remind myself of my own feelings. i also had a friend ask me for in memorizing subjunctive verbs so we came up with our own little reminder. we came up with svamm which means subjunctive verbs affects my mood and it has a bunch. we have also started to study together!