Does back-shifting with "have" give perfect properties

Question

It is a simpler version of my previous question in which two native speaker gave opposite answers. The question consists of two parts to make it more complete and thorough.

1. The keys must have been found.

Does this statement mean that we have the keys now and they have not been lost again? In other words, does have been here bring any properties of the perfect aspect?

2. The keys are sure to have been found.

Which sentence is it equal to?

=2a) I'm sure the keys have been found.

=2b) I'm sure the keys were found.

Answer 1

The keys must have been found.

Note that "must have" (active) or "must have been" (passive) is used for expressing deductions and conjectures: logical necessity, or conviction.

• Bob must have found his keys because I saw him drive out of the parking lot." [I didn't see the keys, but I can deduce they were being used.]

• You must have been a beautiful baby" [I am guessing that you were a beautiful baby also, since you're beautiful now.]

• She must have landed in London by now. [Without direct evidence, but judging by the passage of time since her departure and the expected flight duration, I expect that she has landed.]

So, "the keys must have been found" expresses the speaker's conjecture that the keys are in a recovered state, or that they were in a recovered state based on an observation. (Since time has passed between that observation and the present moment, it may not be true now):

• Just now, I saw Bob drive out of the lot, so he must have found his keys. [Bob is probably still driving and so must still have the keys.]

• Last Friday I saw Bob drive out of the lot, so he must have found those keys he had been looking for. [Current status of the keys is completely unknown.]

The keys are sure to have been found.

This is like like "the keys must have been found" when "must have" is being used to make a conjecture, not a deduction. There is no difference between:

I'm sure the keys have been found.

I'm sure the keys were found.

Watch out though: these two sentences have a possible interpretation of certainty, as in "I'm positively certain that the keys were found". The sentence "The keys are sure to have been found" does not have a possible interpretation of certainty. Therefore, only one of these sentences makes sense:

• {The keys are sure to have been found * | I'm sure the keys have been found}, because I saw Bob bring them to reception, where they were placed in the lost-and-found box! Go take a look for yourself.

"Must have/have been", "sure to have/have been" can never express certainty based on a direct observation. "Must have/have been" can express certainty, but only based on an indirect deduction:

• The keys { must* | } have been found because I see them on your desk.

There is no difference between "was found" and "has been found", due to the semantics of "to find" (in the sense of to recover something lost, not necessarily in the sense of to uncover a truth). Of course, that does not hold for all verbs.

You may be required to use one form or the other depending on the surrounding context, but there is no nuance that separates them. For instance the "long been" construction:

• Did you ever find your wallet?
• Oh that! That {has long (since) been found | was long (since) found * | was found long ago}.

The correct variants say exactly the same thing.

Answer 2

Depending on the context, all of your examples could mean exactly the same thing.

The difference between 1 and 2 is just that you can use 1 when it is apparent that the keys are found. For example, you encounter the now unlocked door or see someone holding the keys, you can say "the keys must have been found." But you definitely wouldn't say "The keys are sure to have been found" because even though it has "sure" in it, it doesn't necessarily mean that the speaker knows that the keys have been found. The speaker just strongly believes that they were found.

Both 1 and 2 could be used in the following scenarios, but 2 could only be used in a scenario similar to this:

"They had the keys when they went in that room, and they have been looking for hours. The keys are sure to have been found by now."

or

"The keys are sure to have been found; my friend is great at finding stuff that people lose."

In both cases, the person is confident that the keys have been found, but doesn't know it. If they wanted to say that they were found, they would have simply said "The keys were found" or more likely "they found the keys."

2a and 2b are essentially saying the same thing. They both express the fact that the keys were once lost and are now expected to be found. The emphasis just changes from the action of finding them to the keys themselves being found.

So, in general, the changes to these sentences are so subtle that they can almost always be used interchangeably. "Have" isn't doing anything significant to change the meaning - it depends almost entirely on the context for which is better to use.