I would use "usted" until she tells you not to OR if she uses "tu" with you.

Just speak to her in English.

We have an older Eastern European nanny and use the formal "you".

Tense is time: past, present, future. You’re asking which level of formality for second person singular conjugations. Do you know her country of origin? Vos is in between tu and usted in formality, but it’s most common in areas that use vosotros instead of usted for plural second person. If vos is an option, use that, if not, ask!

Hi - our nanny speaks Spanish as her primary language, and I've taken several years of college Spanish but am not fluent.

What verb tense should I be using with her? In Spanish class, we always used "Usted" when talking with adults, but I was a lot younger then so not sure if I should still use Usted with adults who are older than I am; I don't want to offend by either seeming overly formal or overly casual.

Thanks for your help!