[mijamo]

It is not that easy and depends a lot on context. I know people who use "usted" to their grandparents (who use "tu" in returns), but at the same time you have lots of shops where they will say "tu" to their customers (particularly clothing, not so much for restaurants). Depends a lot of the country/region and even family.

[jack2312]

Thank you to those who responded! If you happen to see this, do you know if the behavior you describe is more common in Europe, rather than the Americas? (Just out of curiosity.) Most of the people I know who speak Spanish are from Mexico, Central America, and the Caribbean (Puerto Rico or Cuba), and occasionally from South America. Rarely do I get to meet people from other areas, so I'm not sure if this is something I've failed to notice, or if it's maybe due to a regional difference.

Also, am I correct in my understanding that prayer is done nearly universally in "tu"? (I know some Argentinos who use "vos" for informal circumstances, but still use "tu" for prayer, but rarely anywhere else.) Could you help me to better understand the reasons behind this?

Thank you again for your response! It's a beautiful thing to have the opportunity to learn about other cultures from the people who live them.