I thought "the many centuries" would be the answer. But was told the the "en" at the end of århundraden is an indication of the plural form, not "the".
Yes, it's because it's an ett word: ett århundrade, århundradet and plural århundraden, århundradena.
With ett words, is there a rule when the plural has an "en" and when the plural is the same as the singular?
Check the plurals lesson - the notes there give great info (or I found a link somewhere else!) Plurals for ett words: end in a consonant - same (e.g. ett djur, flera djur) end in a vowel - add -n (e.g. ett äpple, flera äpplen)
Flera means several, while många means many. I believe it's similar to english in that they can be interchangeable here.
I think århundrade is just a compound of år and hundrade which both look like their English counterparts year and hundred which makes it way easier
Numbers are much later in the course, so I haven't learned the word for hundred yet, but at least the Swedish -> English part is easy enough, hundrade is close enough to English hundred or German hundert.
Because it makes so much more sense that referring to a century by the number of the previous one! :p
But seriously though, it's just the way Swedish works.
Think of it as how while you can say for example the 19th century, you can also say the 1800s to refer to the same century. Swedish has an equivalent to the second, but not the first.