when is nosotros and when is nosotras used for "we"?
Nosotros is the pronoun for 1st person plural, "we" if you like, when "we" refers to a group of men. Nosotras is used when "we" refers to a group of women. When "we" involves mixed genders, i.e., men and women, nosotros is used. So long as there is a single man in the "we", it becomes nosotros.