The explanation taken from my grammar book says
In a setence which includes a time clause with since, we generally
prefer a past simple verb in the time clause and a present perfect
verb in the main clause. The time clause refers to a particular point
in the past.
- Since Mr Dodson became president unemployment has increased. (rather than … has become…)
However, there's an exercise in another unit that says otherwise. This is a sentence in a problem in the unit and the words in bold is the key answer:
Correct the present perfect continuous verbs using either the past simple or present perfect.
It's remarkable to think that since 1950 influenza (1) has been
claiming (has claimed) more than 50,000 lives in this country,
and in 1957 alone around 6,000 people (2) have been dying (died).
As the book explains, then why not:
It's remarkable to think that since 1950 influenza claimed more than 50,000 lives in this country,
and in 1957 alone around 6,000 people have died.
Best Answer
For tenses, there is never a die-hard rule which applies in every situation. I think the best way you can get used to different tenses is really just to see them in lots of different examples and you try to gain a natural feeling for them.
But, it's important to note that the present perfect describes actions that are finished. In the first example, it is implied that Mr Dodson is still the president (or still the president at the time in which the sentence is concerned) so you use the past simple. However in the second example, those 50,000 lives are dead and done, the action there has terminated and hence the present perfect is preferred.