Learn Dutch: Wij zwemmen hier niet, laat staan dat we die levensgevaarlijke haai zoeken. - We do not swim here, let alone look for that life-threatening shark.

The word dat acts as a subordinating conjunction here. It turns the second part of the sentence into a subordinate clause, which forces the conjugated verb (zoeken) to the very end.

Is dat always required after laat staan?

Only when you are introducing a full clause with its own subject and verb, as is happening here with we ... zoeken. If you were simply contrasting two nouns or short phrases without verbs, you would just use laat staan (for example: Wij zoeken hier geen vissen, laat staan haaien).

Why must the first clause contain the word niet?

The phrase laat staan (dat) translates to 'let alone (that)'. Because it is a negative alternative conjunction, it builds upon a negative premise. You must have a negative word like niet, geen, or nooit in the first clause to logically set up the comparison.

How is the word levensgevaarlijke built?

It is a compound adjective made from leven (life) and gevaarlijk (dangerous), joined together by a connecting -s-. The final -e is the standard adjective ending, added because it modifies the noun it precedes (die ... haai).