答案：A

考查倒装句。句意：直到离开家，他才开始意识到这个家对他来说是何等的重要。not until引导的从句置于句首时，主句需用部分倒装，同时not要置于until之前。由于主句和从句的动作几乎同时发生，故用一般过去时。题干还原为：He didn't begin to know how important the family was for him until he left his home.