"Fathers and sons"
Translation:Fedre og sønner
'faren' already means 'the father'. 'fathers' would be 'farer' if it were following this rule, but 'farer' does already have another meaning: 'dangers'.
en far - faren - fedre - fedrene (father)
en fare - faren - farer - farene (danger/risk/hazard)
There is a slight difference in the pronunciation of 'faren' and 'faren', as the stress is either on the second or both vowels.