This one confuses me. Why does "anka" change to "ankor"? If -en words the end in -e always take -ar at the end why does the 'a' at the end of "anka" change to 'o' in "ankor"? Why doesn't is just become "ankar"?
In the Tips and Notes of the "Plurals" lesson there's a scheme for forming plurals.
All en-words ending on ~a take ~or in the plural: en anka (a duck) - ankor (ducks) - ankorna (the ducks) en kvinna (a woman) - kvinnor (women) - kvinnorna (the women)