For me both "Before, he ate the strawberry" and "Before that, he ate the strawberry" sound at least kind of ok. (I typed "He ate the strawberry beforehand", and that was accepted).
But "Before he ate the strawberry" without the comma sounds like it is referring to something happening before the strawberry gets eaten. "Before he ate the strawberry, he washed it." This is opposite to the meaning of the German sentence, which implies something happening after the strawberry gets eaten.
I also translated the German to 'Before he ate the strawberry' thinking it could be in answer to a prior question such as 'when did he brush his teeth?' :-D There's not enough context here to anticipate that they were looking for 'before that' as if someone were giving a narrative of the life of the strawberry eater.
Can somebody explain why "He ate the strawberry previously" is not correct? One of the suggested answers, "He ate the strawberry prior to", is certainly not good US English, and I've never heard that construct in any of my fairly extensive contacts with UK English speakers. "Prior to" implies something else follows. Additionally, "previously" was one of the options for translation of vorher.
As discussed in the comments above, because "before" in English can be both an adverb and a preposition, when you say "before he ate the strawberry" it seems you are constructing a subordinated clause, as in: "before he ate the strawberry, he went to the gym". The German word "vorher", instead, only means the adverbial "before", meaning beforehand, before now.
"Bevor er die Erdbeere aß (, arbeitete er)." = "(he worked) before he ate the strawberry."
"Vorher aß er die Erdbeere." = "he ate the strawberry before(hand)."
With all respect, I'm still in favor with 'Ealier'. Anybody can argue that "Voher doesn't mean Earlier". OK. But I learned that translation entails division of concentration between the original and the target language,in order to have a balanced translation which is accurate in the original, accurate in the target.
In a case like the example, some flexibly is preffered.