当这猫走进来的时候,那只狗在睡觉.The dog was sleeping when the cat was coming.

The dog was asleep when the cat came.
The dog was sleeping when the cat was coming 不太对,应该改为 The dog was sleeping when the cat came.was coming有过去将来时的含义,如果是现在时态,is coming 是就要来了的意思,
例如,如果有人跟你说:hurry up.你可以说 I'm coming 的意思是我就来了（不要着急）