$11$. were waiting - arrived
$12$. was swimming - saw
$13$. was playing - started
$14$. were driving - stopped
$15$. was blowing - went
$16$. had left - became
$17$. was looking - didn't find
* Diễn tả một hành động xảy ra thì một hành động khác chen vào.
Cấu trúc: When S + V (past simple), S + V(past continuous).
Hoặc : S + V(past continuous) when S + V(past simple).
