"One" should be "the women" or "those women" or something similar rather than "they" since "One" only applies to a group of woman.
"One" applies to a feminine group? but there are other feminine words besides women, couldn't they be girls? This is the feminine version of the pronoun "they", you cannot replace that with something more specific. Sorry when we translate to English that information (that they is feminine) is simply lost, just as when we translate the many forms of "you" from another language and they all become simply "you". In English, that information was just not considered important enough to keep separate pronouns to distinguish between. (Once upon a time we used to have the singular "thou" as well as the plural and formal "you", but now we just say "you".) Scroll all the way down because "one" is also used for animals and things and "oni" can only be used for people with at least one male person in the group: https://www.duolingo.com/skill/pl/Basics-2
you are right the correct rule would be: if at leas one male is in the group use "oni", otherwise use "one"
"oni" means that there is at least one man among "them".
"one" means that "they" are only women.
"ciasteczka" is the diminutive form, actually. "ciastka" is the basic one. It was the course creators' choice to teach "ciasteczka", but I personally find it surprising.
And the diminutive form is used for what? Is like one of the cases of Polish? Sorry if I sound dense.
Deminutives exist in English too, even though they are rare.
- Pig -> piglet
- Leaf -> leaflet
As you can see, the deminutive is a "small-version-derivative" of the original noun with a similar meaning. But it's impossible to guess what the exact meaning will be after it's deminutivized.
Why does it expect the word cookies? In standard english the word is biscuits, not cookies, which is an American word. Why is it not clever enough to allow both?