"We talk about my car."
Translation:Wir sprechen von meinem Auto.
Von is always followed by a dative, über is a "two-way" preposition - if it's used to state a (static) position, it's dative ("Über den Wolken", song title...), otherwise it's accusative, like here. One more example:
- Ich hänge die Lampe über den Tisch ("over the table", acc., motion/destination)
- Die Lampe hängt über dem Tisch ("over the table", dat., position)
However, in the sense of "about", über is always followed by an accusative, like here.
Why not "um" (in its meaning of about) instead of "von"? Is the 'about' meaning of "um" related more to location/proximity perhaps?
"Wir diskutieren mein Auto." should be accepted. This has been reported. 041518.
This might be technically valid, but more than unusual in German. Better with "über", and I don't understand why one should switch from talk to discuss. For me they have some different implications or connotations.
About "etw. diskutieren" without "über": This is used in combination with abstract terms like Fall, Entwurf, Plan, Situation, Werk, Roman, Gedicht, but not with concrete things like a car.