Final answer:
The correct verb form to complete the sentence is "será"
Step-by-step explanation:
The correct verb form to complete the sentence would be será.
The sentence translates to "Before she was an executive, and starting tomorrow she will be the circulation manager."
In this case, it indicates a future action that will take place, so the correct verb form is the future tense, "será" in Spanish.
Learn more about verb forms in Spanish