So, "Do not wake the bear who sleeps" - but why is it "den björn" and not "björnen"? Sorry, my Swedish grammar isn't that great, but I thought we only needed "den" when an adjective came before the noun, so like "Den dålig dagen" = "The bad day", but because in this case we are saying "the bear who sleeps" (rather than "the sleeping bear")... do we still have to use "den", or is that just the way this expression is?
Well, it seems not only to be a fixed expression. If I remember another discussion right the -en is missed out here because of the relative subordinate clause beginning with "som".
I can't find anything about it on Google, but my grammar book has (without pointing it out) a similar example: De barn som jag redan gett biljetter kan går. (I would have expected here also barnen as it's definite plural...)
Infinitive is the un-conjugated verb form: As 'bear' is the third person even English would normally conjugate the verb: The bear sleeps.
As you don't say 'I let the bear sleeps' - the infinitive is even in English obvious. (Keep in mind: The Swedish 'att', the English 'to' or the German 'zu' is not always necessary to indicate the usage of the infinitive)