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The book "Advanced Grammar in Use" third edition, on page 22, says:

"We can use the future perfect continuous (will have been +ing) to say what we think was happening at a point in the past:

Motorist Hesketh WILL HAVE BEEN ASKING herself whether speed cameras are a good idea after she was fined 100 pounds last week for driving..."

So, can "will have been +ing" be an equivalent to "must have been +ing", when we try to stress a possibility that something may have happened in the past?

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  • Yes. Both are equivalent; both mean 'It's a near certainty that ...'.
    – Edwin Ashworth
    Jul 6 '15 at 22:11
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    @Edwin Ashworth - I trust your skills, but I must say it's a jarring construction to me, I associate 'will have' as future, and 'been asking' as past. My inclination is to say, 'Motorist Hesketh would have been asking herself whether...' When would the OP's example be preferable?
    – user98990
    Jul 6 '15 at 22:23
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    @Little Eva It's probably less jarring (in fact it's quite idiomatic) this side of the Atlantic. Huddleston {English Grammar: An Outline} gives a related example: He will have finished by now. (= He's bound to have finished by now.) It's the epistemic modal usage of 'will'
    – Edwin Ashworth
    Jul 6 '15 at 22:31
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    @LittleEva, the perfect tense communicates aspect first, and only expresses past time in relation to context. The modal will projects the context into the future, and the perfect have been simply places the action any time before that future context. With reference to a past event, the expression she will have been asking can be understood as: She was asking, is asking, and will keep on asking [until a time she might be done asking]. I'm not exactly sure what Brits are thinking when they use that construction, but it seems like a roundabout way to express near certainty.
    – ScotM
    Jul 7 '15 at 1:08
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    Will and must are just two modal auxiliaries, and they have a number of meanings and uses, some of which are quite similar. This is not surprising; the same can be said of can and might, for instance.
    – John Lawler
    Jul 7 '15 at 1:38
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First of all, in this sense WILL and MUST are pretty much equivalent. The latter is stronger and in more frequent use. (Of course, WILL is also often used to form the future tense, but that is not how it is used here.)

Next, it's worth distinguishing between WILL BE and WILL HAVE BEEN. Let's look at the sequence of events:

  1. She was fined.
  2. She started asking herself whether cameras are a good idea.

In this case, it would be correct to say, "She WILL [or must] BE asking herself whether...." Here, WILL is a speculation about the present, equivalent to "She is probably asking herself...."

However, if you consider the sequence of events to be three rather than two:

  1. She was fined.
  2. She started asking herself whether cameras are a good idea.
  3. The newspaper article was written.

then "She WILL HAVE BEEN asking herself whether..." is the right way, because it is a speculation about the the past (i.e. before the article was written), equivalent to "She has probably been asking herself...."

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