首先可以肯定:have decided 不是主将从现结构。have decided为but引导并列句后面的宾语从句的主句的谓语动词(使用现在完成时结构)。that从句为have decided的宾语从句内容，that宾语从句你嵌套一个unless=if not连词引导的条件状语从句。
【分句(1)】He was hoping to go abroad
【分句(2)】but his parents (have decided) that they won't support him unless he can borrow money from the bank.【连接词but引导转折关系并列句】
备注:but为并列句连接词，引导转折关系并列句。分句(1)和分句(2)为转折关系
【宾语从句(1)】That (连接词)//his parents【主语】// have decided【谓语动词】//that they won't support him unless he can borrow money from the bank【宾语从句】//.
备注:that为宾语从句连接词，引导名词性陈述分句(that宾语从句)。
【条件状语从句(2)】They【主语】// won't support【谓语动词】// him【宾语】// unless he can borrow money from the bank【条件状语从句】//.
=They won’t support him if he can’t borrow money from the bank.
备注:unless为条件状语从句连接词，引导条件状语从句(unless=if not)。
He was hoping to go abroad but his parents (have decided) that they won't support him unless he can borrow money from the bank.
参考译文(1):他希望出国,但他的父母决定，如果他不能从银行借到钱，他们将不会支持他。
参考译文(2):他希望出国,但他的父母决定，除非他能从银行借到钱，否则他们不会支持他。
【1】was hoping 表示“过去想法/愿望”【描述过去情况】
【2】have decided 表示“现在的决定”【描述过去情况对现在影响和结果】
【3】won’t support 表示“将来的支持”【描述将来事件】
【4】can borrow表示“将来的能力/可能性”【描述将来事件】
以上个人理解，仅供参考。
