Another Spanish Translation
I know that 'tu' means 'you' and that 'su' means 'its/her/his/their'.
I also know that ¿Cuando es tu cumpleaños? means 'When is your birthday?"
The translation says that ¿Cuando es el cumpleaños de su madre? is the correct way to ask what your mother's birthday is. Why is it 'su madre' and not 'tu madre'?
3 Answers
"tu" means "you", but it means "you" in the familiar sense. "Tu madre" means your mother if you're talking to a friend.
"Su" is the pronoun that refers to "usted" which is a more formal use of "you", a person older than you, or someone you don't know well. So"su madre" is "your mother" if you're talking to the person you don't know well.
"Su" is also the pronoun that refers to "el" or "ella", so "su madre" is also "her mother" if you're talking about Juanita's mother.
Hope that's clear!
Wendy
"tú" means "you" and "tu" means "your" (informal)
Hello llamalover, 
You asked,
The translation says that ¿Cuando es el cumpleaños de su madre? is the correct way to ask what your mother's birthday is. Why is it 'su madre' and not 'tu madre'?
Well, this is pretty close, but you need to say:
¿Cuándo es el día de cumpleaños de su (tu) madre? = When is your mother's birthday?
If you were an adult asking about the birthday of someone's mother and you did not know the person well you would use Su which corresponds to the polite form: Ud (you) shown in the example above!
If , however, you knew them well or were a child asking another child about their mother's birthday you would use the familiar form tu instead!
ex: A child might ask his / her friend in this way: ¿Cuándo es el día de cumpleaños de tu mamá (o mami)? = When is your mum's birthday?
I hope this helps.
