Laura will ihrem Freund beim Kochen helfen.
In this sentence ihrem is used instead of ihren. Why is that? There is no dative preposition here.
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Not only prepositions but also verbs determine which case to use in German.
While most of the verbs in German require accusative case, there are several verbs that require dative case and even some that require genitive case.
Helfen is one of the verbs that require dative case.
If you look up a verb in dwds.de you will find a lot of example sentences. That might help you to figure out which case to use with a verb.