Learn Dutch: Wij klagen over de voortdurende overlast. - We are complaining about the constant nuisance.

The word voortduren is a verb meaning "to continue" or "to last". By adding a -d, we turn the infinitive into a present participle (voortdurend), which acts like an "-ing" word in English (continuing/constant). Because it's being used as an adjective in front of a de word (de overlast), it gets the standard adjective ending -e.

Since this lesson is about using infinitives as nouns, could we use het klagen here?

Not in this exact sentence, because klagen is acting as the main conjugated verb here ("We are complaining"). However, you could definitely use it as a noun (a substantival infinitive) to talk about the activity itself! For example: Het klagen over de overlast helpt niet (The complaining about the nuisance doesn't help).

Is klagen over a fixed combination?

Yes! When you want to say you are complaining about something, you always use the verb klagen with the preposition over. Learning verbs with their fixed prepositions is a great habit at this level.

What kind of nuisance is overlast?