could 用于虚拟语气

与以上各条所讨论的 could 的用法不同,上句中的 could 是虚拟语气,即 could 表示反事实的含义。上句的意思可以解释为:If this grain were not used to feed animals, it could be used to feed humans.(如果这些粮食不是用来饲养动物的话,本可以用来供人食用的。)(参见 subjunctive mood $^{3}$ )下面是 could 的类似用法:

1. If you tried, you could do that work. 如果你努力的话,你是能够干这个工作的。

2. If you needed assistance, I could do something. 如果你需要帮助的话,我是可以效劳的。

3. ____ more attention, the trees could have grown better. A. Given B. To give C. Giving D. Having given(全国高考,90-34)