Hi, My daughter had to answer the question "Why is the Eucharistic Prayer the most important prayer of the mass?
She answered "Because when the priest blesses it the Bread and Wine become the Body and Blood of Christ.
the teacher said this was not a complete sentence. I can figure out why. Is it because she started with "because"? thanks for your help
Yes. In spoken conversation, it's not unusual to begin a sentence with "because," but it's not necessarily proper English. In this case, your daughter should have restated the question in her answer:
The Eucharistic Prayer the most important prayer of the mass because.... etc.