21. It is very kind of you to help me. Thanks.

22. We have to wear uniform at school.

23. Don't forget to lock the door before leaving

24. They didn't come here yesterday.

25. They are cutting down many trees now.

26. They are interested in collecting stamps.

Cấu trúc: • have to + V nguyên mẫu

               • before + V_ing

               • interested in + V_ing 

` 21 ` : to help

`  -> `  It's kind of sb + to V

`  22 `  ; wear

`  - `  Have / has to + V nguyên mẫu

`  23 `  : to lock

`  - `  Don't forget + to V : đừng quên làm 1 việc gì đó

`  24 `  : didn't come

`  - `  QKĐ phủ định : S + didn't + V nguyên mẫu

DHNB  Yesterday

`  25 `  : are cutting

HTTD : S + be + V-ing

DHNB : Now

`  26 `  " collecting

`  - `  Be interested in + V-ing