Hi Brett
There is a lesson about this difference planned by the team. In the meantime I can tell you that adding "de" changes the meaning. Take these two examples:
La tienda debe abrir a las 9 de la mañana (The shop must open at 9 in the morming) (obligation)
La tienda debe de abrir a las 9 de la mañana (The shop will probably open at 9 in the morming) (supposition/assumption)
Saludos
Inma