That depends on the original word. I think most words ending in -e you just add the "r" to make it plural, i.e. kvinne = kvinner (woman = women) eple =epler (apple = apples). If it ends with anything other than -e then you add the "er", i.e. gutt = gutter (boy = boys) vin = viner (wine = wines). But like all rules there are exceptions that we just need to try to memorize.
There are different uses for both.
Dere = You (plural) Dere er menn. You (all) are men. This one is used if you are addressing a room full of men.
De = they De er menn. They are men. This one is used if you are addressing one person and pointing to a room full of men.
English used to have two pronouns for "you". They are Thou and You or sth
Is there a different pronunciation of "ikke" depending on whether its referring to a male or female noun?
- dere - you (plural)
- de - they
Dere er ikke menn. (You are not men.)
De er ikke menn. (They are not men.)
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