`1.` The telephone was invented by Graham Bell many years ago. 

`2.` I was sent this letter by Mary two days ago.

`3.` A strange man was seen in my garden ( by me ) yesterday.

`4.` These cars were made in the USA ( by them ).

`5.` Many trees were planted in the school garden ( by us ).

`6.` I wasn't helped with my homework ( by him ).

`7.` Some comic books were bought for Lan's brother ( by her ).

`8.` A letter was received by my sister yesterday.

`9.` I wasn't helped with my homework ( by him ).

`10.` Some comic books were bought for Lan's brother ( by her ).

`11.` A letter was received by my sister yesterday.

`12.` The new pupil was told where to sit ( by them ).

`13.` The room was filled with smoke.

`14.` Two criminals were arrested by the police last night.

`15.` The machinery wasn't damaged ( by them ).

`16.` We were asked many difficult questions ( by him ).

`17.` I was advised to reuse milk bottles ( by her ).

`18.` English was taught to her sister ( by her ) last year.

`19.` The window was opened by Mary.

`20.` Was this circle drawn by Sue `?`

`=>` Câu bị động thì Quá khứ đơn `:`

`(+)` `S` `+` was `/` were `+` `V_{pp}` `+` ( by O ) `+` `...`

`(-)`  `S` `+` was `/` were `+` not `+`  `V_{pp}` `+` ( by O ) `+` `...`

`(?)`  Was `/` Were `+` `S` `+` `V_{pp}` `+` ( by O ) `+` `...` `?`

`=>` Lưu ý `:` Chủ ngữ chung chung như People, someone , somebody, everyone , .... `=>` Lược bỏ ở phần By O

`=>` Trạng từ chỉ thời gian đứng sau By O , trạng từ chỉ nơi chốn đứng trước By O 

$\textit{Note}$

( + ) : $\text{S}$ + was/were + $V_pp$ + ( by O ) + ...

( - )  : $\text{S}$ + was/were + not + $V_pp$ + ( by O ) + ...

( ? )  : Was/Were + $\text{S}$ + $V_pp$ + ( by O ) + ... + ?

$\longrightarrow$ Nơi chốn + by O + thời gian

$\longrightarrow$ Chủ ngữ không xác định như He, She, We, They, People, Someone, Somebody, Nobody, ... $\Longrightarrow$ Được phép bỏ ở phần by O

@quene_danie