Answer

$1.$ Rice and corn are grown in tropical countries.

- Bị động HTĐ `( + )` S + is / am / are + V3 / PII ( by O ).

- grow - grew - grown

- You / We / They + are 

$2.$ Our hand should be washed before meal.

- Bị động ĐTKT `( + )` S + ĐTKT + be + V3 / PII ( by O ).

- wash - washed - washed

$3.$ That cake was made by Linh yesterday.

- Bị động QKĐ `( + )` S + was / were + V3 / PII ( by O ).

- make - made - made

- I / He / She / It + was

$4.$ Their paintings haven't been finished by the student.

- Bị động HTHT `( - )` S + have / has + not + been + V3 / PII ( by O ).

- finish - finished - finished

- I / You / We / They + haven't

$5.$ This thing isn't made by anyone.

- Bị động HTĐ `( - )` S + is / am / are + not + V3 / PII ( by O ).

- make - made - made

- He / She / It + isn't