Does the fast begin one hour before Mass begins or one hour before actual Communion?
One hour before actually receiving communion.
Can. 919 §1. A person who is to receive the Most Holy Eucharist is to abstain for at least one hour **before holy communion **from any food and drink, except for only water and medicine.
The only problem here is you are not sure at exactly what time Communion is. Depends on the priest, it may be 45 minutes into the Sunday Mass, or maybe 25 minutes into it.
As a practice I go 1 hour before the Mass itself. Or if I’m attending morning Mass, I don’t have anything at all in the morning.
Ditto. No confusion there.