Sie hat an der Rede zum 50. Geburtstag von ihrem Vater geschrieben.
what does "schreiben an" here mean?
I am a bit older and for me, the sentence does not sound good. I would rather say it in the following way:
- Sie hat an der Rede zum 50. Geburtstag ihres Vaters geschrieben.
Vater should be genitive here. But nowadays the genitive case seems to disapear from the German language. But for me it sounds better using the genitive case.
schreiben an means here not to participate on or hear a speech, like Eduard wrote. It means, that she worked on the speech or simply, that she wrote the speech.
an etwas schreiben = to be writing something.
It implies that she didn't finish the writing during the time period you are speaking about, but just that she made a bit of processing.
Kind of like "she was working on the speech" -- shows that progress was being made but there was no completion.