Question

12. What.........you.......last weekend?

did - did
did - do
do - did

19. Where did you ...............this book?

buy
bought
buyed

20. Who............that car?

bought
did buy
buyed​

Answer 1

12. What did you do last weekend?

19. Where did you buy this book?

20. Who bought that car?

Quando o verbo vem precedido do auxiliar no passado (did), o verbo continua na sua forma do presente!