could you swim when you were eight years old

题目

could you swim when you were eight years old
为什么这里的were不用原型,could不是情态动词吗?

答案

额,这是个复合句啊,所谓复合句就是俩个句子由一个连词合并为一个复杂句.
因此,会分成主句和从句.而主从句分别有各自的主语,动词.
这里Could you swim是主句
you were eight years old是从句,中间由when连接.
因此,could只能管前面的swim,管不了后面的be动词