The use of tu & su
Any help gratefully appreciated.
When I am talking to a friend about her son do I say
tu hijo
or
su hijo
It's just occurred to me that possibly the tu form might be informal and su the formal term. Is this correct?
2 Answers
You're quite right 
tu = your (informal) su = your (formal) as well as their
Don't forget that both forms take an 's' when speaking of plural things.
Tus hijos. Your sons. Sus hijas. Your daughters.
And tú (you) can be differentiated from tu (your) by the lack of an accent, although the pronunciation is the same.
Also, it would depend if you in general address your friend with tú or with usted (if you are friends, then you should probably address her with tú). If you usted them, use su for possessives. If you tú them, use tu for possessives. But, you have to be consistent with it...otherwise, she could misunderstand you a bit. If you address her as tú and use verbs conjugated in the tú form with her and then say "su hijo" she'll think you are referring to someone else's child.
I think, as a rough rule, you usted people that you would normally address with "sir" or "ma'am" in English. Otherwise, it's tú.
So, I would usted strangers (regardless of their relative social/economic level), superiors, people who wait on me in business establishments (unless I become a regular and get real chummy with them), teachers, doctors, judges, boss's, etc.
I would tú family, friends, and people I interact with frequently in a casual way which may include some co-workers, some classmates, some neighbors, etc.
But that's me...and I've never interacted with any Spanish speaking people except on these forums.
But, your friend...most likely you'll be addressing her with tú...so, I recommend saying "tu hijo."
