present perfect for future

[bokonon]

Hi there!

I was solving a test from my textbook, when I came accross something that I found rather confusing. Here is the example:

By the time you get there, I'm sure he will have arrived.
By the time we have finished, everybody will have eaten.

Is there a reason why the first sentence uses simple present (get) and the second present perfect (have finished)?

Further on in the same textbook there is an even more confusing example:

She is certain to have got there before everyone else.

The answer key offers these sentences as the only correct solutions. Why can't we say it like this:

By the time you've got there, I'm sure he will have arrived.
By the time we finish, everybody will have eaten.
She is certain to get there before everyone else.

Is there any rule explaining why we sometimes use simple present and sometimes present perfect in seemingly identical situations?

[Jamie (K)]

In the examples you gave, the first clause is the time clause. Time clauses start with time expressions, such as "by the time", "when", "after", "before", "as soon as", etc. In time clauses the future tense cannot be used, so we use one of the present tenses to express it.

The tense in the second clauses is not the present perfect; it's the future perfect: "will have arrived", "will have eaten". It's basically the present perfect preceded by the modal "will".

The present perfect tense is not used to express the future.

[bokonon]

[Jamie (K)]

That should teach me to try to answer questions like that on only 4 1/2 hours' sleep a night for two days in a row. I missed your question, and you caught me in a mistaken, ill-advised statement. What I had in mind, in my sleep-deprived state, was that the present perfect isn't one of the tenses formally taught for describing the future

You're right that in, "By the time we have finished, everyone will have eaten," the present perfect indicates a future event.

There are a couple of ways of looking at it, but since I've never, ever seen a rule, I'm going to have to make up something ad hoc. This looks like one of those things that grammar books don't cover.

One way to view it is that what appears to be the present perfect in that sentence isn't really, but is simply a reduction of the future perfect. In that sense, we can think of time clauses as not requiring a present tense to express the future, but as requiring deletion of the verb that indicates the future. In other words, the future is not expressed in a time clause as [+present], but as [-future].

So a reduction of the future tense that way results in a present tense, and the reduction of the future perfect results in the present perfect. Therefore you can get, "By the time we have finished..." expressing a future event. It's future perfect minus future.

It's possible to say both of these:

By the time we finish, everyone will have eaten.
By the time we have finished, everyone will have eaten.

They don't have different meanings, but they conjure up a slightly different image in the native speaker's head. "By the time we finish..." produces a mental image of the people at the moment they're finishing. "By the time we have finished..." gives an image of the people after they've finished and are moving on to something else. If the people are finishing Thanksgiving dinner, for example, with "By the time we finish..." I see them putting the last morsel of food into their mouths. With "By the time we have finished..." I see them already having left the table and turning on the football game on TV.

Sometimes the present perfect is used in places where another tense would normally be used, but the speaker or writer wants to give the impression that something has definitively been completed. Some of these usages don't seem to appear in the grammar books, and I even checked with the syntax goddess of the linguistics department I studied at, and she didn't know anything about it.

A few months ago we had a big argument on this site about whether you can use the present perfect in a clause beginning with "since", where "since" does not mean "because". For example:

The car rides better since I bought new tires.
versus
The car rides better since I've bought new tires.

You don't find the second usage in places like the British National Corpus, but you will hear sentences like this on the street, and plenty of them will come up in a Google search. It appears that many people use the present perfect to stress the perfective aspect of the event, that it was finished some time ago and was not repeated.

I think the present perfect in those future time clauses also reflects a wish to stress the perfectiveness of an action.

[bokonon]

Thanx! That clears it up for me. That's what I call an answer As for the other thing -

[Jamie (K)]

[smart_man]

[smart_man]