Questions & Answers about I det øjeblik hvor han kom, begyndte det at regne.
Why is hvor used here instead of når or da? Doesn't hvor mean "where"?
You are right that hvor usually means "where". However, Danish also uses hvor to mean "when" directly after a noun that describes a specific point in time, such as øjeblik (moment), dag (day), or år (year). You cannot use når or da to link back to a time noun in this way.
Why is the phrase det øjeblik instead of den øjeblik?
The word for "moment" is a t-word in Danish: et øjeblik. When you want to say "that moment" or "the moment", you must use the matching t-word form for "the" or "that", which is det.
Why does the main part of the sentence say begyndte det instead of det begyndte?
This is the Danish V2 (verb-second) rule in action. The entire first part of the sentence (I det øjeblik hvor han kom) counts as the first block of information. Because the main verb must come second, the verb begyndte comes right after the comma, swapping places with the subject det.
Can I drop hvor and just say I det øjeblik han kom?
Yes, you can! Just like in English where you can say "the moment he came" instead of "the moment when he came", Danish allows you to drop the connecting word hvor in this sentence.