参考答案：had been

解析：

考点：虚拟语气。此题考查与过去事实相反的假设，主句用“should (would) have+过去分词”，条件从句用过去完成时。

[例句] If my lawyer had been here last Saturday，he would have prevented me from going．

如果我的律师上个星期六来，他就不会让我去了。