why in this case the article "o" is not needed? like for example: Eu nu sunt o femeie
If want to say you are something or someone (i.e you are part of a group), you do not use an article. Example:
(Eu) sunt student. - I am a student.
(Tu) ești inginer. - You are an engineer.
There are exceptions to this, but this is the general idea.
I'm still confused. Does this mean there is a subtle difference in meaning between "eu sunt femeie" and "eu sunt o femeie"?