"Good fathers have long days."
Translation:Bonaj patroj havas longajn tagojn.
Its no a common phrase in the US. Maybe they are tying to say a dad that works a long day is a better provider.
Ne, -n indicates the object. You will almost never start a sentence with a noun or adverb ending in n.
I thought that subjects never take -n. -n marks (among other things) the object and it's description. But your wright , I'll hardly ever use -n at the start of a sentence.
Same reason longa needs to be pluralized - an adjective attached to a plural noun gets the plural suffix.
I still don't understand why the start of the sentence's plurals end with j but then they end with jn
There's no verb at the front, just 'good fathers'. In the second half it states that they have something. (Long days) That something is an object --> needs an -n. (and a -j, 'cos it's plural!)