"De jongen was van zijn paard gevallen."
Translation:The boy had fallen off his horse.
We don't use 'hebben' with 'vallen', so it's always 'was gevallen' (or 'is gevallen').
My little brother used to say it like this (so there's some sense of logic to it), but unfortunately it's not correct. You could say though: "De jongen was van zijn paard af gevallen". I think it's because of the word vanaf - it seems that the part "af" is optional. Isn't that also the case in English? Like, this sentence is short for "the boy had fallen off of his horse"?