i knew the robots could play chess with us变疑问句

I knew the robots could play chess with us.变疑问句
Did you know the robots could play chess with us?
你知道机器人能和我们下棋吗?
否定句：I didn't know the robots could play chess with us.
变否定句,加了助动词did后,knew由过去式变回原形.而宾语从句中的could仍然要与主句的时态保持一致,并不因为加了did,就改成can.否定句呢，为什么could不变成can刚才在上面说了一些。这个didn't 只能让主句的动词knew变回原形，而从句的could并不受这个助动词的影响。