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Without taking risks, nothing gets started.
Literal
Adventure [object-を] don't-do-if, nothing not-begin.
始まる ('to begin, to start') is the intransitive partner of 始める — used here for 'nothing begins by itself.' The proverbial frame 冒険しなければ何も始まらない pairs the conditional ~なければ ('if not') with 何も+negative ('nothing'), yielding the familiar 'nothing ventured, nothing gained' nuance with the slight twist that without daring, no story even gets going.