He left home as a young man and (not see) since th

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hasn't been seen
年轻时他就离开了家,自从那时起就再也没人见到他.
======以下答案可供参考======
供参考答案1:
hasn't been seen 当他年轻的时候离开家，并且从那时起从未被别人看见过
供参考答案2:
He left home as a young man and (hasn't been seen) since then
他小时候就离开家了，从那以后再也没有人见过他
供参考答案3:
He left home as a young man and had not been seen since then. 或者简单一点
He left home as a young man and not seen since then.
他年轻时就离开了家，从此再没有人见过他。