问题: 这是什么虚拟语气？
But for the help of their group, we would not have succeeded in the investigation.
解答:
but for可以理解成 要不是因为..；多亏了..
But for the help of their group, we would not have succeeded in the investigation.
要不是因为他们的帮助，我们不会取得这次调查的成功。
这里是表示对过去事实相反的假设虚拟
if条件从句用 had + 过去分词 ，主句用 would/should/could/might + have +过去分词
本句可以改成
If we had not had the help of their group(从句），we would not have succeeded in the investigation.（主句）
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