"Her father is tall."
Why is it wrong to say "Il suo padre è alto"? Or more generally: when is it okay to just write suo/mio/tuo/... withot article and when do you have to say il suo/la tua/...? I don't really see a concept there, yet, but sometimes it is counted as correct and sometimes it seems to be a mistake both to put the article there or to leave it.
When some close family members (unmodified and in the singular) are preceded by a possessive the article must not be used: padre (father) is one of those, although for some reason papà (dad) is not (it can still be omitted though). Then there's the rule that when the possessive agrees with the subject it can be switched with the definite article, so "suo padre è alto" and "il padre è alto" have more or less the same meaning.