In English we can say:
They had been delivered the old gears.
Why can that not be said in German with:
Sie waren die alten Zahnräder geliefert worden?
The use of liefern remains mysterious to me. In English to say that something is supplied is no different than to say it was delivered, since a supply necessitates some type of delivery. But in German liefern seems to be making a distinction depending upon how it is used, and I do not understand that distinction.
First, note that in English:
When a verb has two objects, either the indirect object or the direct object of the active verb may become the subject of the passive verb.
– Collins
In German, however, only transitive verbs, that is verbs that take the accusative case, can form the passive the same way as the passive in English.* This is because the accusative object becomes the subject in the passive voice.
Let's look at the active voice for your English sentence.
The delivery man [subject] delivered the old gears [direct object] to them [indirect object].
Now let's look at the German translation:
Der Lieferant lieferte ihnen [wem?, dative object] die alten Geräte [wen?, accusative object].
Because "ihnen" is in the dative it cannot become the subject in German when using passive voice.
Nonetheless, we can make the accusative object "die alten Geräte" the subject, if we want to using passive voice:
Die alten Geräte [wer?, subject] wurden ihnen [wem?, dative object] vom Lieferanten geliefert.
Thanks to flexible word ordering in German we can say:
Ihnen [wem?, dative object] wurden die alten Geräte [wer?, subject] vom Lieferanten geliefert.
Note that we have rearragned the component parts of the sentence, but their function has not changed.
*Note that some transitive verbs still do not allow the passive and the Unpersölicher Passiv is a construction for expressing the passive voice with dative objects.
