Hello JedBeyer, I am here to answer your doubt about this Dutch sentence. If we take into account that the translation of the Dutch word "een" means a/an ( an indeterminate article). It "only" combines together with a noun phrase (determinate + noun) for instance: " een appel" (an apple). You cannot see it making the role of an adverb of frenquency. So, the correct sentence in Dutch should be: "per dag." I hope to help you in something :D Have a nice day!