'Will' or simple present: Why there is a difference in these two sentences?

Question

Consider these two sentences from the book Grammar in Use:

1. Alex will look after our cats while we're away next week.
2. The new drug goes on sale in the USA next year.

As you see, the authors of the book recommend to use "will" in the 1st sentence, because there is "less routine arrangement" and present simple in the 2nd because it's a "fixed event".

I do not see any difference between these two sentences from this aspect. Both events are planned and will take place only once in future.

The book says that 'will' is also useable in the second sentence, but then why is present simple not possible in the first one?

Answer 1

English, like other languages, uses tense semiotically as well as semantically. Thus, when context clearly establishes the time in question, we can use tense to signal something about the focus of our remarks. The so-called historical present is an example.

I have found no discussion of the historical present being used to bring the future into tighter focus, but there is no reason why is shouldn't be, and so it is. If important things happen in 1848, they can happen in 2048. How would we describe the future portrayed in science fiction? The world is at war or peace, money is or isn't still a thing, etc. History is a work in progress, after all.

Like the traditional historical present, using the present tense to talk about the future helps focus our attention on things right before us:

"We are lucky to have Fred join us; he is well-educated, accomplished, and starts on Wednesday."

Note how I have treated "starts on Wednesday" as if it were an adjective. I am not trying to communicate something about the timeline, viz., that he won't be here until Wednesday. That intention would call for the future tense. Rather, I am trying to communicate something about the new guy that explains why we are lucky today, viz., that he will be here so soon.

Answer 2

The phrase while we are away next week is a specific situation not a "general state of affairs", and for that reason we would not use the simple present.

Alex looks after our cats whenever we're away. ^OK

Alex looks after our cats while we're away next week. ^{not quite idiomatic*}

Alex is looking after our cats while we're away next week. ^OK

Alex will look after our cats while we're away next week. ^OK

We use will + bare infinitive or is + participle if we wish to express the idea of an isolated event or action in the future, that is, something that is not on a timetable ("The train arrives in Berlin at 6AM tomorrow" or "the drug goes on sale next year").

*Not unless Alex's care of the cats is being cast as part of a timetable or as part of a plan or arrangement made in advance:

While we're away next week Alex looks after our cats and John brings in the mail, and I will mow the lawn before we leave.