Who's functions

I'm Studying English and I need to know how does "who" work as an object? And as an subject?

Thank you

Who saw you? (subject)
Who did you see? (object- also, note that in formal language some would use 'whom' here')

In addition, you always use the object form whom with a preceding preposition:

I just don't know to whom I could turn if I were in trouble.
or
I just don't know who(m) I could turn to if I were in trouble.
;-)