"Aunt has a bat."
Translation:Dì có một con dơi.
It's a classifier. Basically, con clarifies that dơi is indeed an animal, and as far as I'm aware it's pretty much required when you're counting a thing (a bat, two bats etc., as opposed to bats in general).
We have also learned the classifier người for persons (like in người phụ nữ) and cái for objects (like in cái ô) so far.
Good question and it may vary from region to region. Generally speaking, however, cô refers to any female older than your mother including any older sisters. On the other hand, dì generally refers to any female younger than your mother including her younger sisters. Both cô and dì can be used to mean Miss or Misses and cô is also used for any of your father's younger sisters, older ones are called bác in the South of Vietnam (this may differ elsewhere).
Yeah, pronouns were rough for me too, when I first started out. But actually, the more you speak/write, the more you get used to it. I started off with the ones I would use most, including anh, chị, em, which I would use most frequently with my peer group. After that, I learned cô, chú, and cháu. It takes some getting used to, but I think I mostly got it.
However, some things still throw me in for a loop lol. In the gay community for example, there are more subtleties as far as choosing who to address as anh/em. But you learn as you go.
Vietnamese is the distinctive language in the world in calling their relatives by listening to their conversations without introductions. Aunt has six meanings in Vietnamese depending on the relationships:
"Dì" is the younger sister of your mother.
"Bá/Bác" is the older sister of your mother.
"Cô" is the younger sister of your father.
"Bác" is the older sister of your father.
"Thím" is the wife of your uncle on your father side.
"Mợ" is the wife of your uncle on your mother side.
Duolingo should accept all variations if they well understand the Vietnamese culture.
I think when speaking generally, indiscriminately, then something like "mèo ăn dơi" is a fine sentence where we're missing both the plural marker and classifier, like the sentence "đàn ông ăn đu đủ". But that would change when we are saying "the men eat the papaya." = "Các người đàn ông ăn quả đu đủ." Otherwise, I am completely wrong haha! But that's how I have been thinking anyway.