(Since + past simple) or (Since + present perfect)?

Question

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.

Answer 1

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.

Answer 2

I think there is some confusion about what is the time clause in the second example.

It's remarkable to think that since 1950 influenza has claimed more than 50,000 lives in this country, and in 1957 alone around 6,000 people died.

Here the time clause is "since 1950". That's all. Nothing more. (Note: it might be more correct to call it a prepositional phrase, but it's clearly the case that "since 1950" is doing the work of a time clause in that sentence.)

There are two independent clauses:

since 1950 influenza has claimed more than 50,000 lives in this country

Here the present perfect is used because it describes something that happened in the past continuing up until today.

in 1957 alone around 6,000 people died

Here the simple past is used because 1957 is a long time ago.

In the first example, a time clause, something that has a subject and a verb, is used in place of a date to emphasize an event which the writer/speaker believes to be relevant to the main clause.

Since Mr Dodson became president unemployment has increased.

The writer thinks that Mr. Dodson's administration has had an adverse effect on unemployment. By the way when did Mr. Dodson become president? Let's say it was 2012. Then the sentence could be written equivalently as

Since 2012 unemployment has increased.

but then it would not be clear to the audience that unemployment increased because Mr. Dodson is a bad president.