Learn Dutch: We merken dat de interesse langzamer afneemt dan dat de onderzoekers dachten. - We notice that the interest decreases slower than the researchers thought.

You use dan to compare simple nouns or phrases. However, when you compare something to a full clause—meaning a phrase with its own subject and verb, like de onderzoekers dachten—you use dan dat. It links the comparison to the whole idea of what the researchers thought.

Why is the verb dachten at the very end of the sentence?

The phrase dan dat functions as a subordinating conjunction. Just like standard conjunctions such as dat or omdat, it forces the conjugated verb in its clause (dachten) all the way to the end.

Why isn't the separable verb afnemen split up into neemt af here?

In a main clause, it would split: De interesse neemt langzamer af. But here it sits inside a subordinate clause introduced by dat (...dat de interesse langzamer afneemt), which pushes the verb to the end. When a separable verb is pushed to the end of a clause, its prefix (af) and the verb (neemt) reattach into a single word.

In English, we often say 'slower than what the researchers thought.' Can I use dan wat in Dutch?

No, this is a common trap for English speakers! While English casually accepts 'than what', Dutch requires dan dat when introducing a full clausal comparison like this one.