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I encountered this sentence when I was learning another language. I have never used such a sentence in English nor seen one, but it seems it exists.

What idea does this sentence trying to convey? What time should we relate to?

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    What time we are talking about? Erm... yesterday? – FumbleFingers Jul 4 at 17:26
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    When we find out the truth of the matter (which will be in the future, because we don't yet know for certain), it will turn out to be the case that she has bought a new phone. It doesn't really make any difference whether the phone is / was / will be bought before or after time of speaking. We know it's before time of utterance in the example as given, because of yesterday. But She'll have bought it when we see her again says nothing about whether the purchase has already been made at time of utterance. – FumbleFingers Jul 4 at 17:36
  • @FumbleFingers Languages that have special inferential moods sometimes use those moods/modes to express the same things as our modal verbs can sometimes do, and as they are doing here via will have bought. – tchrist Jul 5 at 14:08
  • @tchrist: Yeah, I realise that my "rationalisation" there is just a [potentially] useful way of looking at things. I recently listened to Sean Carroll's "morality" podcast, where he makes much of the fact that only humans can envisage unreal scenarios. But people often want to deliberately obfuscate the exact nature of "unreal" references, rather than put them up for full logical scrutiny. So over time we end up cooperatively moulding our languages to better support "vagueness" wherever "precision" isn't actually essential (i.e. most contexts) – FumbleFingers Jul 5 at 14:44
  • This usage is much more common in British English. She'll have bought a new phone yesterday, I expect. [as opposed to some other time].No one seems to have pointed that out. – Lambie Jul 6 at 15:34
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Days of Future Passed

In English we normally call this construction the future perfect. It’s a construction that represents, if you would, the very opposite of the more familiar and rather more frequently used future-in-the-past time shown in the second clause here:

  1. Yesterday they told her that it would rain today. [=future in past]

The opposite of a time representing the future in the past is necessarily one representing the past in the future, like this:

  1. By the time she gets here later this afternoon, it will have rained already. [=past in future]

The first sentence takes place in the past (they said) but it references a time frame that was still in the future of that past-time reference frame. It had not yet happened then, and we don’t know whether it has happened now or even whether it for certain shall.

The second sentence’s first clause is grammatically in the present tense (she gets) but it nonetheless represents a future time (later this afternoon). Its second clause’s verb is a perfect infinitive have rained used with the modal verb will, here in its epistemic to indicate a future time.

So the perfect aspect makes it a completed one, and the will marker moves that completion into the future reference frame.

Inferential will

Sometimes will be means “probably is”, and will have been means “probably was”.

The sentence She will have bought a new phone yesterday means that she “surely must” have done so yesterday. This is not the normal past-in-the-future the way the future perfect normally works out to meaning so much as it is a probable state of some past event; a probability statement, if you would.

Yesterday specifies when the action was completed, and here the modal verb will means mere likelihood not actual futurity.

Consider this simple example where will means probability not futurity:

Jack: There’s someone at the door!
Jill: That will be Billy.

That means it must be, or “has to” be Billy. It’s about a probable present time, not about a future time — despite that will.

When you want to to express that same thing in the past, you use a perfect construction; you don’t just change will to would because that just weakens the probability.

Jack: There’s someone at the door!
Jill: Would that be Billy?

By backshifting the will into would, you have made it less probable. You haven’t changed it to a past time. For that, you need a perfect construction to show that it has already been completed:

Jack: Somebody called yesterday but didn’t leave his name.
Jill: That will have been Billy.

That means it must have been Billy who called yesterday. It’s still the future perfect construction, but this time it doesn’t refer to something that hasn’t happened yet. It refers to something that has very probably already happened — yesterday.


In Related Languages

English’s cousin languages and its slightly more distant European relatives also have the same concept of a past in the future; they just call these different things, but most form them in essentially the same way.

  • The Latin future perfect uses a simple tense¹ in the active voice, but in the passive voice uses a compound tense.² Examples are videro (I’ll have seen) versus visus ero (I’ll be [or have been] seen).
    1. =a single-word, synthetic tense via inflection of the root verb’s stem.
    2. =a multi-word, analytic tense via an inflected auxiliary plus the passivized verb’s past participle.
  • The French call these future perfect constructions the future anterior, or in French futur antérieur. Examples include j’aurai dit (I’ll have said) and j’serai parti (I’ll have left).
  • Italian similarly calls it the future anterior (futuro anteriore).
  • Spanish calls it either the future perfect (futuro perfecto) or the compound/composed future (futuro compuesto), the latter making clear that it is a compound verb construction not a simple, one-word tense. Examples include habré dicho (I’ll have said) and habrá llovido (It will have rained).
  • In Portuguese it is usually the future perfect (futuro perfeito).
  • In German it is (often called) the second future (Futur II).
  • In Scandinavian languages it is the “exact” future (futurum exaktum).

Using the simple future to mean the probable present (or non-past) and using the future perfect to mean the probable past is not a use of the future which is peculiar to English alone; Spanish and Portuguese can also use their own futures, including the future perfect, to indicate probability.

For example, if you in Spanish/Portuguese say Será un/um amigo, even though será is the literal future tense meaning “will be”, it can instead mean that it’s “very likely/probably” a friend, or it “must” (epistemic sense) or “has to” be a friend.

This shows how even languages with actual future tense morphological inflections (unlike English) can still use these future forms with a non-future sense: a probable present situation.

Once you move further away from English than its Germanic and Romance cousins, you begin to see the same sentiment of inferred probability expressed differently. The Balkan languages have such things as the “renarrative mood” and the “inferential mood”.

So English modal uses of (inferred) probability like...

  • That will be John. (probable/inferred present)
  • That must be John. (probable/inferred present)
  • That will have been John. (probable/inferred past)
  • That must have been John. (probable/inferred past)

...would in some of those languages be translated using special inferential moods that their verbs can take on.


Summary

All these languages, including English, are more flexible than a simple one-to-one mapping between verb tenses and times referenced would allow for.

  • This doesn't explain that 'yesterday' though, does it? – TonyK Jul 4 at 23:40
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    @TonyK You’re right, it doesn’t. This is the future of probability, which is not the normal future. She will have done it yesterday means that she “surely must” have done it yesterday. Yesterday specifies when the perfect action was completed, and here the modal will means likelihood not futurity. This is not a use of the future that is in any way unique to English; Spanish and Portuguese are also able to use their future inflections to indicate probability, both simple futures and compound futures as we have here. Será un/um amigo can mean that it must be or probably is a friend. – tchrist Jul 5 at 0:28
  • So your answer is irrelevant? – TonyK Jul 5 at 0:47
  • @TonyK No, I've updated it. Thank you for pointing that out. – tchrist Jul 5 at 0:50
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This is a rather peculiar construction, and I don't think the other answers have quite got to the point. Here is an example, without the future perfect:

Kid: Dad, when is granny coming?

Dad: She'll be on the train by now.

It means something like "It is very likely that she is on the train by now", or "It is to be expected that she is on the train by now." It is a perfectly sound construction, but I would not expect to see it in formal written English.

So your sentence just means "It is very likely that she bought a new cellphone yesterday", or "I assume that she bought a new cellphone yesterday."

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    I believe you are right about what it means, although I’m not sure that the asker was aware that this is what they were asking about. :) “Somebody’s at the door!” “That will be John.” is the same use of the future to mean a probable present. It means that right now it surely is or must be or probably is John, not literally that at some future time it shall be John. He’s already here, after all. English modals are really sneaky, aren’t they? :) – tchrist Jul 5 at 0:32
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We should also pay attention to the basic meaning of the modal "will", that of "volition" (willingness, intention and prediction based on it). My girlfriend had been long craving after a mobile phone and yesterday she told me she was heading to a shop. Now I am almost sure she's bought it. The possibility comes up to 90%. "She will have bought the phone yesterday" conveys the idea of strong probability based on the girl's intention to by a cellphone. Thus the situation itself brings down to us her plans to buy a phone.

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There are two components to this time expression.

  1. When she bought the phone (yesterday)
  2. What time she is expected to possess the phone (either now or at a point in the past or future)

The first one is obvious: she is expected to have bought the phone yesterday.

As for the second one, she is expected to have the phone in her possession—for whatever usage is the subtext of this communication—for some future or present or even past requirement.

In other words, the speaker might have been asked if the person in question will have the phone currently or in the future, or for something between now and the time she should have purchased the phone. (Example of the latter usage: "Does she have her new phone yet? I tried to call her an hour ago but she appeared to be offline.")

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    I'm not sure all the stuff about have = be in possession of is relevant here. It just looks to me like a straightforward auxiliary / helper verb (along with will) indicating future with a close relationship to the past. That "future" being when we find out for sure about something that's currently just a strong prediction. – FumbleFingers Jul 4 at 17:41
  • @FumbleFingers: I never suggested that the have is anything but a component of the future perfect (I've now switched that word to possess in order to avoid any misunderstanding). The idea of possession comes from her having bought the damn thing. I have anticipated several scenarios for which the question would have been a valid response. You certainly can't mean she doesn't possess the phone, can you, or that such possession is not germane to the issue at hand? – Robusto Jul 4 at 18:54
  • She'll have bought the phone yesterday, or even the day before, but it probably won't be delivered until next week. In this age of Internet shopping, I don't automatically link buying with having, being in possession of. – FumbleFingers Jul 5 at 11:45

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