When do I use tu or usted
How would I speak to each person below? Which one is tu or usted? Dr. Hackett? Susana a four year old grandfather the governor my best friend my sister the principal a classmate a stranger
4 Answers
Dr. Hackett --- a doctor, a respectable person you would show respect to -- usted
Susana, a four year old --- I'm guessing you are older than 4, so tú is appropriate when speaking to a young kid.
Grandfather --- your grandfather? If so, I'd go with tú because you have a close relationship with them
The governor --- again, someone you'd show respect to, so usted
My best friend --- someone you are close with, so tú
My sister --- see best friend
The principal --- someone you definitely want to show respect to! usted
A classmate --- someone your age, most likely, so tú
A stranger --- when first meeting, use usted
It varies with region, to some degree. Some places use usted with almost everyone (including parents and grandparents). Other places use tú with almost everybody.
It is almost universal for parents to call children by tú, though. (In most places, but not all, children also address parents and grandparents by tú).
For most places, anyone who is not your friend or close relative, you use usted with. Friends and close relatives get the tú treatment.
You can also use tú with non-friends and non-relatives with people you wish to sound disrespectful to. For example, a mugger would probably address his victim by tú. If someone cuts you off in traffic, you will address them by tú, though probably from inside your car where they can't hear you.
Usted is a similar to using sir / ma'am in English, by the way (not exactly, but fairly so).
One thing I've seen in movies that surprised me a little was teachers calling students by usted, which I guess demonstrates the professional relationship there.
On the other hand, a master/sensei type person (like a Jedi Knight, for example) might call his pupils by "tú" while they will address him/her by "usted." This appears to be true regardless even if they consider each other friends.
God, oddly enough, is generally addressed by tú in most regions. I imagine the fact that He is considered Father of all overrides the fact that He is also consider King of kings. But I only theorize there.
I agree with Sonrisa:
Grandparents are close relatives and deserve the love and the closeness of "tú."
As I see it, grandparents aren't friends; they deserve all our love and respect. So if I were you I would use usted and ustedes when talking to them. and I would often visit them. 
