The general rule is that you do not use or offer the 'tu' to anyone except small children until or unless you are invited to do so.
I have found over the years that once an older person 'offers' me their first name as opposed to me calling them signor, signora or signorina then that is the signal to move to using 'tu'!.
Many (if not all) Italians do accept that with foreigners there is often some confusion and mis-use of the 'tu', but this is generally shrugged off. Offence is rarely taken and indeed this will often lead to being offered the 'tu' then and there....
But if the daughter has written and offered the 'tu' already then you can do the same.
I asked my Italian teacher (an Italian native). She said that with family members "tu" is fine. However, she also said (if I understood her correctly) that if the family is from Sicily or far south, "voi" is a better choice. To me this doesn't quite make sense--isn't "voi" the plural familiar? She seemed to be saying that "voi" was more formal.
I'm just starting to learn Italian--so I may not have understood her correctly.
Poipuo4 wrote:I want to write to an older cousin (old enough to be my father). Do I use "tu" or should I be more formal? His daughter is my age and uses "tu." However, this will be my first letter to him.
the "tu" is correct with a cousin, and any family member, the "voi" was used in the fascist age especially, and in the deep south italy, but now is rarely used ," voi" is rarely used with subordinate persons, the "lei" with no family members, so if you write your cousin, you can write:
ti scrivo questa lettera per dirti... etc... and it is right! regards, suanj