Why does this sentence start with "Det" and not with "Den"? if we are talking about a book ( en bog) shouldn't we use "Den" in the beginning of the sentence? Thanks in advance :)
That is a very frequently asked question :)
Until the gender of the noun is revealed, you use 'det'. For example:
A: Det er en bog
B: Den er god
B uses 'den' because A established the gender of the noun and can therefore use the article 'den' to refer to 'bogen'