因为宾从用的是一般讲来时" they won't support" 所以主句要一致，用现在时。由于decide发生在过去，所以这里用现在完成时，讨论的是decide对现在的影响，因为讨论的时间点是现在。
如果后面从句里都是过去时，那主句就可以用一般过去时了，讨论的是整个发生在过去的决定：
He was hoping to...., but his parents decided that they wouldn't support him unless he could borrow...
具体哪道？好几道呢呀
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