`1,` Volunteers often clean up the dirty river to protect the environment.

`@` often -> DHNB thì HTĐ

`@` to + V-inf: Để ... -> Chỉ mục đích

`2,` Volunteer students enjoy cooking meals for the homeless people.

`@` enjoy + V-ing

`3,` We provided evening classes in our hometown for 20 street children last year.

`@` last year -> DHNB thì QKĐ

`4,` The social workers deliver meals to the elderly and sick people every Sunday morning.

`@`  deliver sth to sb: giao cái gì cho ai

`5,` We can make our city greener and more beautiful by planting trees and flowers in public areas.

`@` by + V-ing: bằng cách làm gì đó 

`@` can + V-inf: có thể làm gì 

Answer:

`1.` Volunteers often clean up the dirty river to protect the environment.

`-` often: DHNB thì hiện tại đơn

`->` `(+)` S `+` V_inf / V_s/es `+` O

`-` S `+` V ( chia thì ) `+` to `+` V_inf: chỉ mục đích ( để làm điều gì đó )

`2.` Volunteer students enjoy cooking meals for the homeless people.

`-`  enjoy `+` Ving: thích thú, yêu thích làm gì đó

`3.` We provided evening classes in our hometown for 20 street children last year.

`-` last year: DHNB thì quá khứ đơn

`->` `(+)` S `+` V_ed / V2 `+` O

`-` provide sth for sb: chu cấp thứ gì cho ai đó

`4.` The social workers and sick people deliver meals to the elderly every Sunday morning.

`-` every Sunday morning: DHNB thì hiện tại đơn

`-` deliver sth to sb: giao cái gì cho ai đó

`5.` We can make our city greener and more beautiful by planting trees and flowers in public areas.

`-` can `+` V_inf: có thể làm gì

`-` by `+` V_ing: bằng cách nào đó 

`#nhp`