书上原句：（海明威老人与海）
a) If I could have used a bat with two hands, I could have killed the first one surely. （虚拟语气）
要是我能用双手抡起一根棒球棒，我准能把第一条鲨鱼打死。
我疑惑的是为什么原句a)中if条件句使用了could have used ? 如何理解？
如果把could have used改成had used,如下b)句：
b) If I had used a bat with two hands, I could have killed the first one surely.
意思有变化吗？