Mañana (yo, hacer) ... muchos deberes.

The sentence is a Spanish phrase that translates to "Tomorrow (I, to do) ... many duties." The blank likely requires the correct conjugation of the verb "hacer" (to do/make) for the first person singular (yo) in the future tense. The future tense of "hacer" for "yo" is "haré". Therefore, the completed sentence would be "Mañana (yo, haré) ... muchos deberes."