Hi! Some nouns in Danish are irregular.

Like Man = mand (plural: Mænd).
Similarly, Duck = and (plural: ænder).

You just have to learn them on the way. :)

Here are some additional Irregular nouns for you.

It's an umlaut. These exist in all living Germanic languages. (Apparently, Gothic may have been the only Germanic language that didn't have them...)

Umlauts have become relatively rare in English, but they have survived in some of the most common words: goose/geese, man/men, foot/feet, mouse/mice. And they don't just occur in plurals: fox/vixen, come/came, fall/fell.

You can think of it this way: An e (or i) in a suffix influences how an earlier vowel is pronounced. The resulting modified vowel may have a pronunciation that is otherwise rare in the language. In the beginning, people wrote this as ae, oe or ue, but later various shorter ways of indicating the changed vowel quality became established, such as a with an e on top, ä or æ. (Or just an e, as in English.)

Yeah, I thought I was having a seizure trying to hear that one.

it depends on the irregularities in the indefinite plural, some words dont take the r, others might have an umlaut (a -> æ)