I thought that when you say 'I am' or 'he is', that this was followed by just plain nominative?
no. there are two ways of translating "she is a daughter"
Ona jest córkĄ.
Ona TO (jest) córkA.
my answer "ona to córka" was not accepted here
Actually, the only time when we do not accept the "X to Y" construction is when X is a personal pronoun. It just seems very clumsy.
Read here: https://www.duolingo.com/comment/16373167 - Part 2.
Każda kobieta jest niejako córką :D