In the book 'Urdu: An Essential Grammar' by Ruth Laila Schmidt, it states on p93:
In what situations would تھا/تھی/تھے be negated with na and in what situations would it be negated with nahiiN? I'm aware the simple past in general is negated with نہ and occasionally نہیں (same book p126), so I suppose that applies to ہوا/ہوئی/ہوئے rather than تھا/تھی/تھے in the case of ہونا? What are the rules for تھا/تھی/تھے then?The negative used with the past of hōnā is nahīṁ (and occasionally na)