A very common word, and it takes the mood tense for whatever you want to happen.
Ojalá deje un mensaje · Ojalá sea pronto
It certainly has something to do with the Arabic word for God, from those eight centuries when southern Spain was part of the Muslim world. There are several accounts of the exact origin; the teacher's favourite is a contraction of "ojo-Allah", eye-God, said when you want something.
I want to tell him to leave me a message becomes I want to tell him that he leaves me a message, with the mood tense.
Without the mood tense you are reporting a fact, not passing on an instruction.
Quiero que me dejes un mensaje · Déjeme un mensaje
Dejar is to leave in the sense of leaving a message, not leaving a room, which is salir.
Cuando te deje el mensaje, llámame · Es esencial que vea mi mensaje
Note the positive command with the little word on the end: déjeme, llámame.
The same shift as -tion to -ción.
-tial → -cial
esencial · espacial
Cuando te deje el mensaje, llámame
Referring to something that has not happened yet, so the mood tense. The teacher's framing again: we feel a bit superstitious about it.
Closing thought on the mood tense as a whole.
So listen out for it, get interested, and ask why it was used each time. That is how the picture fills in.