"He did not understand what had happened."
Translation:Nechápal, co se stalo.
The issue is, that rozumět is usually used with an object in dative and I like "Rozuměl tomu, co se stalo." much more. But the valence dictionary does explicitly list "rozumíš už, co se stalo?" as an example, so I will add your answer.
And actually, we also have a typo in our answers, so we do not currently accept even "Nerozuměl tomu, co...", I will fix that too.