could you swim when you were eight years old

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?

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