Help me understand

Hola Michael L

Thank you for your question! The phrase "ustedes van a casa de su hijo" means "You (plural) go to your son's house", referring to more than one person. In contrast, "usted va a casa de su hijo" would mean "You (singular, formal) go to your son's house", addressing only one individual.

The lesson uses "ustedes" because it refers to multiple people going to the son's house. Changing it to "usted" would alter the meaning, as it would then apply to only one person.

I hope this helps clarify the difference! Let me know if you have more questions. 

Saludos

Silvia