If I recall, it's because in Danish, adjectives after a definite article end with an -e whether it's singular or not. Examples: Den triste bog Den store kat Det smukke hus (sg.) De smukke huse (pl.) If it were the indefinite article, then it would be "en trist bog"
The general pattern seems to be to make the adjective in a "definite article–adjective–noun" construction grammatically plural, even if the noun is singular.
surely one cannot say in english: I write the sad book. To me that doesn't sound correct. Shouldn't it rather be: I write a sad book?
The situation I can imagine is: I write the sad book, my sister writes the happy one :D
It's perfectly grammatical; one could say it, but would probably never want or need to.
It could also translate to English as "I am writing the sad book." in which case it sounds reasonable.
You certainly can say that in English. "Which book are you writing? The sad or happy one?" "I write the sad book". It's a little awkward but not incorrect.
Same meaning in French too. Some dictionaries say this word's origins are from Old French.
Tristis means sad in Latin so I would imagine that's probably a common ancestor
Ind vores univers, der er to bog; en trist og en glad. De er viktige at holde verdenen i balance. Mikkel skriver den triste bog, og Karl skriver den glad en.
Apologies for crappy grammar, some (most?) of that came from google translate :P
Please don't use Google translator for text nor grammar! The only way is to use it to get a little bit of the topic of the text. Or else you can be misunderstood. If you translate to my language "I'm a hard rock fan" you'll get smth. like "I am a ventilator of the firm stones". ;-)
Google translating your sentence into google Danish results in "Jeg er en hårdrock", i.e. I am into hard rock. Your result sounds much interesting, though :-)
I put the unhappy smile at the end of the sentence and it was marked as wrong...
Yes, Duolingo will mark some punctuation marks as incorrect. Parentheses are included as part of that.