Whoa, I do remember that scene now. And I read the book in German, so the word may have been really popular once... PS.: The films aren't that bad, if you are able to shut up the part of your brain that's always comparing them to the book while watching. PPS.: I love the word; it takes my irrational fear of those foul beasts away and makes me smile :)
When referring to something unknown, we default to the neuter version.
At the point where "det" is used in the sentence, the reader/listener still doesn't know that "det" is going to be a spider. If they did, then the sentence would be a bit pointless.
"Det (introducing an unknown) er en edderkopp. Den (pointing back to edderkopp) er farlig."
As a general rule, the vowel in a word with a single consonant is a long vowel, and the vowel in a word with a double consonant is a short vowel. Bear in mind that "long" and "short" in Norwegian isn't identical to "long" and "short" in English. Here are some examples:
Maten er varm. (The food is hot.) - The long
a in maten is drawn out a little: ma-aten.
Matten er ren. (The mat is clean.) - The short
a in matten is said quickly.
Fint, takk! (Fine, thanks!) - The
a in takk is pronounced quickly.
Fint tak! (Nice roof!) - The
a is drawn out.
There are, of course, exceptions, including til, om and hvis. They're not drawn out.
"This" is used to refer/point to things that are closer to you, and "that" is used for things that are farther away, either physically, emotionally or figuratively.
"This cup (over here/in my hand) is blue, while that cup (over there) is blue."
"This bicycle is mine; that one is yours."
"I think this idea is better than that idea."