19, My mother (usually / drink) .................... a cup of coffee every morning to start her day, but recently she (switch) .................... to green tea as a healthier alternative. 20, By the time the police arrived, the burglars (steal) .................... valuables from the house for over an hour.

`19.` usually drinks - has switched

- DHNB : usually - every morning 

`->` HTĐ ( + ) S + V ( s/es )

`->` Chủ ngữ số ít `=>` V s/es

- DHNB : recently 

`->` HTHT ( + ) S + have/has + P2

`->` Chủ ngữ số ít `=>` has

`->` switch - switched - switched

`20.` had stolen

- By the time + QKĐ , + QKHT : Hành động xảy ra trước hành động QK khác

`->` QKĐ : S + V2/ed

`->` QKHT : S + had + P2

- steal `->` stole `->` stolen