Some reasons to use The passive voice in Vietnamese
language:
1/ Subject (the actor) is not important / obvious/
unknown
Con sư tử đã bị giết
The lion was killed. -> We don't know who killed
the lion (Unknown), or the person who killed it was not important in our
sentence/conversation (Not important)
Căn phòng đã được lau dọn
The room was cleaned -> We know the person who
cleaned the room because he/she does it every day (Obvious)