"Une femme agit seule."
You mean in a sense of "looking/appearing lonely," right...? I'd say: La femme a l'air solitaire. Frankly, I doubt anybody would ever use the sentence we just went through in any context. This has to be some computer-generated nonsense like many other sentences. Google can't find a single sentence like that.
you miss "acting" "agit" is not translated by "is" but by "is acting" = continuous present of verb "to act" In French there is no continuous present, so to simplify, Duo generally translates the English continuous present by the French simple present: "elle agit" = she acts or she is acting.