Learn English – ‘could have + past participle’ to talk about possible events in the future

futuremodal-verbsperfect-aspect

BACKGROUND

In this earlier thread, Edwin Ashworth approved a use of 'could have + past participle' for the future event that was precluded by context as in:

(1) Mary could have arrived tomorrow, had she managed to get that flight.

But he did not approve a use of 'could have + past participle' for possible events in the future as in:

(2) */?Mary could have arrived tomorrow.

Nor did he approve a use of 'may have + past participle' for possible events in the future as in:

(3) (?)Mary may have arrived by next week.

(4) *Mary may have arrived tomorrow.

I think that there is no reason to treat 'next week' and 'tomorrow' differently insofar as both refer to a future time. So the different treatment for the latter two examples is because of the existence of the preposition 'by'. That is, inserting 'by' would somehow increase the acceptability at least for the 'may have + past participle' construction.

QUESTION

My question is whether inserting 'by' would increase the acceptability of the 'could have + past participle' construction as in (2):

(2') Mary could have arrived by tomorrow.

Note that (2') is along the lines of (2) in that Mary's arriving tomorrow is a possible future event as opposed to a future event precluded by context as in (1).

ADDITIONAL EXAMPLE

An example of the 'could have + past participle' construction being used in a possible future event similar to (2') is found in this Harry Potter:

Malfoy could have attacked half the Muggle-borns in the school by then!

EDIT

I'll have to admit that it was not easy to find an example of the 'could have + past participle' construction being used in a possible future event. So maybe it's not that idiomatic to use the construction for a possible future event. Does that mean that (2') as well as the Harry Potter example is somehow unnatural?

Best Answer

The construction you're suggesting might be reasonable in certain contexts. The HP sentence would seem natural in the context of a discussion about a specific time in the future (the ides of March) or if Malfoy was being compared with someone else. Why wouldn't the simpler "Malfoy could kill..." be used? Because the perfect construction carries with it the idea of accumulation. Your sample sentence: "Mary could have arrived by midnight," doesn't sound natural to me (without a negative context), but "Mary could have walked ten miles by the morning," does sound natural, and it likely has to do with the idea of something accumulating.