Question

but for,wish等词的虚拟语气中从句谓语是什什么

Answer 1

1.but for your help,i couldn't have done it. 要不是有你的帮忙，我就不能完成这件事情。这里介词短语but for your help是假设的条件，可以还原为条件状语从句，if there had been no help from you, i couldn't have done it. 类似的介词还有without
2.虚拟语气用在“wish+宾语从句”结构中，表示很难实现或不能实现的愿望，可译为
“但愿。。。”“悔不该。。。”等。表示现在不能实现的愿望，从句的谓语动词用过去式；表示对过去的愿望，从句谓语动词用had+过去分词或could+have done ；表示将来的愿望；从句的谓语用would/could+动词原形。
例如：i wish it were spring now.要是现在是春天就好了，事实是冬天，是对现在的虚拟
i wish you had not bought that house 我真希望你没买那所房子，对过去的虚拟，事实是已经买了
i wish you could/would celebrate the Lanter Festival with us 要是你能同我们一块过元宵节就好了（由于某种原因不可以） 对将来的虚拟
刚学的，希望对你有用

Answer 2

but for = Were it not for: except for: 后跟名词或名词性词组. 如跟从句, 应为宾语从句, 其谓语用正常时态.
We would have reached the summit but for the weather. 如果不是这天气, 我们已上到顶峰了.
We would have reached the summit but for what he did. 如果不是他做的那件事, 我们已上到顶峰了.

wish 的虚拟语气中从句谓语用过去时:
I wish you were here.
I wish you could have seen it.