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. 再问: 否定句呢,为什么could不变成can 再答: 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。