Why "arhundraden" and not "arhundrader"? Isn't "er" (or "ar") the plural suffix?
århundrade is an ett word, so it works the same way as ett äpple.
ett århundrade, århundradet
plural: århundraden, århundradena
So is there a difference between flera and många? I used flera here to check and it was correct...
Flera is two things:
The comparative of många (many) meaning more. This can also be fler: ”Jag har fler(a) hundar än du.” = I have more dogs than you.
A word on its own meaning several, a number of. This can only be flera: ”Jag har flera hundar hemma.” = I have several dogs at home; I have a number of dogs at home.