"It is the man's book."
Translation:Det är mannens bok.
It is hard to explain, but I'll give it a try. (Someone else, please help me!)
The difference is pretty subtle but I'll give two examples. First off, yes, bok is an -en word so you should use den together with it. But this is a different case:
- "Vems bok är det här? Det är mannens bok" (Who's book is this? It's the man's book)
- "Vilken är mannens bok? Den är mannens bok!" (Which one is the man's book? That is the man's book) (Said while pointing at it)
I think the first one uses det because we are talking about something without specifying what it is first, so we don't know whether to use den or det. At least that's how it feels to me. Hopefully someone can expand upon my answer
You've pretty much said it. Using 'det' in this context is called the formal subject, and it's used when the real subject of the sentence is postponed - i.e., pushed later into the sentence. So, as you said, we don't yet know what's being talked about, so we always use 'det'.
It's difficult to explain with a sentence like "It is the man's book", because that is the only available construction - "The man's book is" doesn't make sense. An easier example would be something like: "A dog is in the kitchen/En hund är i koket". If we reconstruct it as "There is a dog in the kitchen" it becomes "Det är en hund i koket", because until the subject "en hund" is revealed to us, how can we know what grammatical gender it has?
It exists in French and German as well - "il y à" always takes the masculine gender, and "es gibt" is always neuter. It's in English too, as "it/there is", although the issue is less clear because English doesn't have gendered pronouns for objects.