Learn German: Ich weiß, dass das Rezept nicht laktosefrei ist. - I know that the recipe is not lactose-free.

In a main clause, nicht often goes at the end to negate the entire sentence. However, this is a subordinate clause starting with dass, which pushes the conjugated verb (ist) to the very end. Since the verb is occupying the final spot, nicht must come before it. Additionally, the rule is that nicht always sits directly in front of the adjective it negates, which in this case is laktosefrei.

Why are dass and das right next to each other here?

It looks a bit funny, but it is entirely grammatically correct and very common! The first one, dass (with a double 's'), is the conjunction meaning 'that'. It introduces the subordinate clause. The second one, das (with a single 's'), is simply the definite article 'the' for the neutral noun Rezept.

If I wanted to say 'I don't know that the recipe is lactose-free', how would the placement of nicht change?

To negate your knowledge rather than the recipe itself, you must place nicht in the main clause. It would become: Ich weiß nicht, dass das Rezept laktosefrei ist. In complex sentences, putting nicht in the main clause versus the subordinate clause completely changes what is being negated.

How does German form words like laktosefrei?