Eight years ago I posted this:
There are two ways of saying the word “you” in Spanish: usted (ud.) and tu. The latter is less formal. The former should be used in work situations, when you don’t know the person, as a sign of respect, and for the elderly.
I didn’t always know this rule and I continue to be horrible about following it. Sometime in college though I figured out I should be using ud. and not tu when talking to grandma. Even her children call her ud. and use that form of the verb with her. So I switched. Gueli (grandma) did not at all appreciate the switch and promptly told me so. I was the only person who continued to call her tu and she liked it.
Last weekend I was at her house. She sat next to me during lunch and I helped to cut up her meat. She chatted to me the entire meal, picking on her children and grandchildren, complaining about the soccer team and her hearing, rearranging her chair time and time again. The whole while, I addressed her as “tu.”