This has already been answered here and (to some extent) here. Also (https://german.stackexchange.com/questions/66006/how-bad-does-it-sound-in-german-not-to-separate-separable-verbs?rq=1 [here]).
– bakuninMay 06 '22 at 06:34
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@bakunin: I agree. To clarify a bit, wenn is a conjunction (not a preposition) which creates a subordinate clause, and all parts of the verb are placed at the end in a subordinate clause. The prepositions here are in and an (part of an dem = am).
– RDBuryMay 06 '22 at 06:54
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This has nothing to with prepositions. As RDBury correctly noted, there is a subordinate clause which demands verb-last order, and separable verbs are not split if they are last in a clause.
– RHaMay 06 '22 at 06:59