How do you use tu formally?
We haven't used it in class in such a long time and I can't remember how to properly use the usted form. Do I replace su and tu with Ud. ? And is it necessary to use Ud. after a verb'
Yes, Doris is correct. If it is obvious, from the context, that you are speaking directly to one person, you don't need to use "usted" with the verb.
You replace tu with Usted; you use su when you mean "your". It's used out of respect to the person it's directed, in Puerto Rico, it's used mostly with older people and with strangers. Ex. Tiene usted la hora? - Do you have the time? Usted se ve muy guapa hoy - You look very pretty today. Su cabello está mojado - Your hair is wet. But I don't think you have to use always after a verb. I guess it depends on the contents of the sentence.