Is there some rule that tells you when to use en and when to use ett? Really annoying....
Nope. There are some patterns but mostly gotta learn the gender of each word. Sorry.
Why is ett anka, flera ankor not a correct answer, the English is one duck ?
Each noun is either an "en-word" or an "ett-word" and anka happens to be an "en-word" :):
Is there a logic in when something is going to end in -er, -ar or like this one -or when transforming into plural?
This is one of those sweet cases where there's actually a decent rule in Swedish plurals. "En" words ending in "-a" are "-or" in plural. Flicka flickor. Anka ankor. Skola skolor.
Why is "en and, flera ankor" correct? Anka - Ankor, And - Änder. They shouldn't be mixed, right?
Yes, they're different. Anka is the domesticated duck.
Why -ett anka - is wrong but en anka is right
Some nouns are ett-nouns (≈ 20%) and some are en-nouns (≈80 %) and you have to learn which one by heart. Have fun :)!
Why is it sometimes "Ankor" and sometimes "Ankorna"?