`20`/ watched ( yesterday `->` QKD: S + V2 + ... )

`21`/ was ( your friend số ít nên + is, last week là thì quá khứ nên is `->` was )

`22`/ were not ( they + are, last night là thì quá khứ nên are `->` were ( not ) ) 

`23`/ was talking ( S + V2 + ... + while + S + was / were + v-ing + ... )

`24`/ was walking. ( S + was / were + v-ing + .... + when + s + v2 + ... )

`25`/ was attending ( nói về `1` chuyện xảy ra trong quá khứ tại `1` thời gian cụ thể thì dùng QKTD( at this time last year ) : S + was / were + V-ing + ... )

`26`/ was standing ( gt giống câu `24`) 

20. watched

yesterday → QKĐ

21. Was your friend 

last weekend → QKĐ

22. weren't

last night → QKĐ

23. was talking

Hành động đang xảy ra trong quá khứ → QKTD

Hành động chen vào → QKĐ

24. was walking

Hành động đang xảy ra trong quá khứ → QKTD

Hành động chen vào → QKĐ

25. was attending

At this time last year → QKTD

26. was standing

Hành động đang xảy ra trong quá khứ → QKTD

Hành động chen vào → QKĐ

Cấu trúc thì quá khứ đơn:

*Động từ thường:

(+) S + V_ed/ V2

(- ) S + didn't+ V_inf

(? ) Did + S + V_inf?

*Động từ tobe:

(+) S + was/ were + ...

(- ) S + was/ were + not + ...

(? ) Was/ Were + S + ...

Cấu trúc thì quá khứ tiếp diễn:

(+) S + was/ were + V_ing

(- ) S + was/ were + not + V_ing

(? ) Was/ Were + S + V_ing?