Breakdown of Warum unterstellst du ihm diesen Fehler bei der Lieferung?
du
you (informal singular)
dieser
this / that (fem.: diese, neut.: dieses)
ihm
him / it / to him / to it (dative)
bei
at, near, with
Questions & Answers about Warum unterstellst du ihm diesen Fehler bei der Lieferung?
Why is the prefix unter- not separated at the end of the sentence?
The verb unterstellen is a variable prefix verb. When it means to accuse or to insinuate, it is inseparable. If it meant to shelter or to store, it would be separable, and you would say Warum stellst du ... unter? In spoken German, the inseparable version places the stress on the root verb (unterSTELLst), while the separable version stresses the prefix (UNTERstellst).
Why do we use the dative pronoun ihm instead of the accusative ihn?
The inseparable verb unterstellen requires a dative object for the person being accused (ihm) and an accusative object for the thing they are accused of (diesen Fehler). You are grammatically attributing the mistake to him.
Why use bei for with the delivery instead of mit?
In German, bei is often used to mean during or in the context of an event. Mit der Lieferung would sound more like the mistake was physically shipped inside the package, whereas bei der Lieferung means the mistake happened during the process of the delivery.
Why is it der Lieferung if Lieferung is a feminine noun?