。
I knew the moment I fell that I had broken my wrist.
Literal
I [topic-は] fell-[past-た] moment [at-に] wrist [object-を] broke-[fact-こと] [subject-が] was-understood.
~瞬間に ('at the moment of, the instant') marks a precise point of realization. The structure is [verb past]+瞬間に — the attributive past verb directly modifies 瞬間 ('moment'). ~ことが分かった ('it became clear that X') uses the nominalizer こと to package the realization into a clause that 分かる ('understand, become clear') can take as its subject.