Question

could you swim when you were eight years old

Answer 1

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