Translation:Why did my grandfather always forget to buy milk?
I get the past tense for the forgeting. However the way i read it with "does", is that he always forgets. With "did", either he no longer forgets. In other words, the act of forgetting is past tense, or, he is no longer, he is past tense. The word "always", IMHO makes the fogetting a past and present activity.
What am i missing