I seem to have missed the rule about when you do and don't use a definite article in front of a noun modified by an adjective. Could someone enlighten me as to why there's no article in this?
It’s mostly used, but it can be left out. I don’t know of an exact rule, but with some adjectives it’s always left out, like samma, hela etc., and sometimes it sounds a bit odd leaving it out. But quite often both work, and here you could say either.
So 'Han är yngste sonen' or 'Han är den yngste son' both work? Like PaulExcoff above, I think I need to go back and review some things. :)
Okay, now I'm confused. "Det är den sämsta bok jag vet" doesn't have a definite form of "bok" (and I thought it was because there would just be one too many defining clauses). So.. which is it really?
You can say Han är yngste sonen or Han är den yngste sonen and you can say Det är den sämsta bok jag vet and Det är den sämsta boken jag vet.
The reason you can say den sämsta bok jag vet is that it's followed by a relative clause ('jag vet' – it would be clearer that it is a relative clause if it had started with som, and it could have, but som isn't necessary). So you could also have said Han är den yngste son jag har if you added a relative clause there too.
Compare with the general rule for adjectives + definite article:
boken = the book
den där boken OR den boken = that book
den röda boken = the red book
den där röda boken = that red book
den (röda) bok … 'the (red) book, …' only used before relative clauses
Swedish used to have masculine and feminine genders, a long time ago. The only remnant from that time is that definite adjectives describing a male can optionally use an -e ending instead of an -a one.