I don't know if this is a rule, but:
One uses "San" when it's a title before the name, like in "San Antonio", "San Pedro", "San Juan"..., with the exception of "Santo Tomás" (I don't know why). "Santa" never changes ("Santa María", "Santa Ana", "Santa Lucía"...)
"Santo" when one is talking about the quality of a person: "Mi mamá es una santa", "Juan tiene paciencia de santo", "El santo padre de la iglesia", "La santa que hace caridad en el pueblo", "Los santos médicos que vinieron a ayudarnos"...
__________________
♪ ♫ ♪ Ain't it wonderful to be alive when the Rock'n'Roll plays... ♪ ♫ ♪
|