Bad Translations of Imperfect Sentences
Imperfect sentences require more context. Also, all translations of "were doing" and "used to do" should be equally correct for almost all sentences.
I agree with GeorgeT. I found the following sentence very curious: "My uncle and his wife used to have three children". So, "used to have" implies that they no longer have them. What happened to them? Now I'm thinking maybe they're running a foster home and they used to have three children, but one got adopted so now they only have two.
I have found fault with other lessons, but this lesson was pretty good.
Many of these sentences are random, and I won't try to excuse them all, but I didn't find fault with "... used to have three children" -- I'd say this if my children have moved out and are living on their own, such as in the conversation: "Have you always had an empty nest?" "We used to have three children".
However, it does seem strange to say "I used to drink yesterday" -- this is not a recurring action if it just happened one specific day. Instead, it's likely an ongoing action, and that's why the speaker used imperfect. I noticed when I answered "was drinking" for these types of sentences, I had no issues. (Though "They used to do the same thing every day." had a specific recurring time period.)