$\text{1.}$ Brenda's family isn't known anything about.

$\text{→}$ thì HTD: S + is/ are/ am + PP + (by)

$\text{2.}$ It is thought that someone started the fire deliberately.

or The fire is thought to have started deliberately.

$\text{→}$ It + be + V1 + that + S1 + V2 + S2.

= S2 + be + V1 + to-v/ to have + PP + (by)

$\text{→}$ nếu cùng thì dùng to-V

$\text{→}$ nếu thì bị lùi 1 thì dùng to have + PP

$\text{3.}$ A doctor should be asked to see that cut.

or That cut should be seen by a doctor.

$\text{→}$ Với Modal Vs: S + modal Vs + be + PP + (by)

$\text{4.}$ It is said that Chris was in the army.

or Chris is said to have been in the army.