What is the meanings of below sentences?
1) Five people were killed in the earthquake.
2) Five people had been killed in the earthquake.
Are above sentence in passive voice?
If yes, then who is the doer I mean earthquake?
Also if earthquake is the doer then why in the earthquake and not by the earthquake?
We don't need the preposition "by" in a passive construction.
Tens of thousands were killed during the civil war.
The tense differences, were killed, had been killed, express a time relationship: the past perfect refers to a time that is earlier than another time also in the past:
Tens of thousands were killed during the civil war.
Tens of thousands had been killed before peace talks began.
Yes, the sentences are passive constructions. You do not have to mention the "doer" or agent in a passive construction.
You could mention that the earthquake is the "doer" by using by the earthquake:
Five people were killed by the earthquake.
See this Google search for the string killed by the earthquake.
You could also use that prepositional phrase in your sentence:
Five people were killed in the earthquake by the earthquake.
This is because in the earthquake does not indicate the "doer", but rather it mentions a weather or disaster event (such as in the storm, in the hurricane, in the fire, etc.). In the earthquake is an adverbial phrase providing more information. You could consider it as saying in what circumstance the people were killed.
My sentence may sound a bit strange with two prepositional phrases using earthquake, but it's grammatical. As is
which is a phrase from a speech by President Abraham Lincoln. It just happens that I've used the same word to describe both the circumstance and the "doer".
For the difference between the past and past perfect, see When is the past perfect exactly needed?.
"Five people were killed in the earthquake" is simple past, and the one I would use in this scenario.
"Five people had been killed in the earthquake" uses past perfect. This is used to refer to if the deaths, etc. occurred before another time in the past. This is a grammatically wrong sentence because there is no other past time specified here.
"Earthquake" is an event, so it doesn't take "by", it takes "in".
