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I've learnt that, to form a passive sentence from an active one, one uses "von" and then one mentions the Subjekt of the aktiv sentence:
Hans isst den Apfel. → Der Apfel wird von Hans gegessen.
(Stylistically not very nice, but grammatically OK, I hope). But one finds sentences like
Irgendwas wird durch das Gesetz geregelt.
which, presumably, is the passive voice of
Das Gesetz regelt irgendwas.
When should I use "von" and when "durch" to form the passive voice?
Both "von" and "durch" indicate the "Täter" in passive. That is, the agent of the passive action.
The Täter becomes the subject when you transform the passive into an active sentence.
The difference between "von" and "durch" is that you use durch when the agent takes an instrumental role.
So, in your two example sentences, Hans actively ate the apple, but the Gesetz was merely the instrument through which the government acted.
This is nicely explained here. (in German but hopefully it is clear).
To summarise that example they first have a mechanic who fixes a motor vs. a therapy which increases the rate of healing. The mechanic takes "von", since they are actively fixing the enginer, which the therapy takes "durch" because it is simple the instrument.
von and durch, but mostly one is more common than the other. The distinction instrumental or not is a guideline, that doesn't do the job all the times. Positive Examples: Hans eating and apples and laws regulating something. Negative Example: Literal Instruments: Der Ton wird durch die Tuba erzeugt. and Der Ton wird von der Tuba erzeugt. Both are equally useable. I guess, it needs a lot of experience with the German language to always be sure about which one to use.
The answer of David Hall - that durch is used in an instrumental way, and that von is merely naming the agent who does something - already has a point.
I just want to add two things:
durch is implying an intention, so that the agent named is actual just an instrumental means of someone.
So
Das Fenster wurde durch den Wind zerstört. [The window got destroyed by the wind.]
sounds a bit skewed, because it slightly implies, that someone used the wind in order to break the window.
A postive example of using durch:
Durch die flächendeckende Impfung sind die Masern zurückgedrängt worden. [By the means of comprehensive vaccination measles have been repelled.]
Here, die Impfung is just the instrument someone used in order to achieve something.
The agent does not need to be a person. agent is a grammatical terminus technicus here and does not refer to a person. So
Das Fenster wurde vom Wind zerstört. [The window got destroyed by the wind.]
is totally fine.
To be honest, "das Fenster wurde durch den Wind zerstört" does sound correct to me. I do not fancy someone using the wind as an instrument when I hear the sentence. Pragmatically there seems just a very tiny difference here between "von" and "durch".
But after sleeping over it, I want to add: The distinction is most useful when you need to add both, 'von' and 'durch', to one sentence:
Das Haus wurde von den Angreifern durch Brandlstiftung zerstört.
The house was destroyed by the attackers with fire.
You can think of it in English like this:
von = by an agent acting directly on
durch = by means/way of or through an instrumentality
The cars were smashed by the hail = Die Autos wurden vom Hagel zerschmettert.
The house was connected to the street by means of/through a driveway= Das Haus war durch eine Auffahrt mit der Straße verbunden.
I get woken by the sunlight = Ich werde vom Sonnenlicht geweckt.
I'll wake myself up by means of/through sheer will = Ich werde mich durch bloßen Willen wecken.
I made it by/through pure luck = Ich habe es durch pures Glück geschafft.
As you can see, it's not always a simple matter of "by" in English either. The passive voice is quite funky.
