Combining past simple and present perfect .

Umed from Iraq writes:
In the news, I heard a reporter from BBC World talking about the casualties of a blast.
"Two have been killed and twenty were injured," he said.
My question is: is it possible to use two different tenses when describing the same event (have been and were) and what is meant by this?
......................................................
Roger Woodham replies:
Have been or were?
Combining have been and were in this way is perfectly acceptable here, Umed. Often, in language use, deciding whether to use the present perfect or the past simple depends on your perspective on the event.

Two people were killed when the bomb went off but the effect of that is still with us - the recent past is connected to the present, so I choose to use the present perfect passive tense. Similarly, twenty people were injured when the explosion occurred - I am thinking of that moment in time in the past, so I choose to use the past simple passive. But their injuries are still evident at the present time, so I could have used the present perfect passive. There are no time phrases in this example to guide us, so any combination is possible depending on whether you are viewing it as a past act or one that impacts on the present:

Two people have been killed in an explosion and twenty have been injured.

Two people were killed in a bomb blast and twenty were injured.

Two people have been killed in an explosion and twenty were injured.

Two people were killed in a bomb blast and twenty have been injured

Of course, if you decide to specify the moment in time when this happened as a fixed point in the past, you are obliged to use the past simple passive:
Two people were killed this morning when the bus they were waiting for mounted the pavement and crashed into the bus stop.
You would then continue with this tense:

Six more were seriously injured and taken to the nearest hospital.
----------------------------------------------------------------

Here are some more examples of the way in which these tenses have been combined and used in press and media reporting over the last few days:

Sea marshes return to Norfolk
Sea defences, which were erected to protect farmland on a mile-long stretch of coastline in East Anglia, have been dismantled in order to return 200 acres to salt marsh.

Here, the past simple passive is used first to describe the erection of the sea defenses at some point in the past and then the present perfect passive to describe their dismantling, the effect of which is still with us.

The Oktoberfest draws larger crowds
Beer consumption and attendance have both risen at this year's Oktoberfest in Munich. The world's biggest beer party has so far drawn 5.1 million visitors and roughly 5.7 million litres of beer have been served since 21 September.

Note that this event was still in progress when the report was made, so only the present perfect is used. Note the use of time adverbials, so far and since 21 September, which are associated with the present perfect.
If it had been written after the festival had ended and was clearly in the past, it would have looked like this:

Beer consumption and attendance both rose at this year's Oktoberfest in Munich. The world's biggest beer party drew a total of 5.9 million visitors and 6.4 million litres of beer were served during the course of the festival.

Knifeman attacks Paris mayor
The Mayor of Paris, Bertrand Delanoe, was stabbed during an all-night party at the town hall yesterday. The suspect, Azedine Berkane, a computer programmer, has been arrested. He was taken into custody immediately and has confessed to the stabbing, according to judicial officials.

Here, we are back to a mixture of past simple and present perfect. The stabbing occurred at a particular point in time in the past, so the past simple passive is used. The suspect is still under arrest, so the present perfect passive is used here. He was taken to the police station immediately after the stabbing - this is a finished action, so the past simple again. He has not withdrawn his confession, so we return to the present perfect here.

TAKEN FROM: http://www.bbc.co.uk/worldservice/learningenglish/grammar/
learnit/learnitv241.shtml