Here is an example:

"What is particularly interesting is that despite being heavily based upon the CPR, the Court Rules do not incorporate the costs management additions to CPR 3 that were introduced following Rupert Jackson’s reforms in 2013. Given the extent to which the rules of procedure otherwise mirror the CPR, that will have been a very deliberate decision."

It seems to me that 'will have been' refers to a past event which is thought of on the supposition. I had a chance to look at "Will Have Done" tense- meaning and examples.

For me, this appears to have similarity with 'must have done' something in the past which gives us an indication of the conclusion being made.

Is there any difference between 'will have done' and 'must have done' in terms of grammar and meaning?

Kind regards,

  • That is just American legal lingo. They mean: it was. It is used when the author does not want to be 100% definitive. It's future in the past.
    – Lambie
    Apr 1, 2019 at 21:37
  • 2
    Legal lingo, yeah, I believe it. But is this American? When I read it, I think "Britain". "Costs management" doesn't sound like one of our expressions. Also, Rupert isn't a real common name over here.
    – Lorel C.
    Apr 1, 2019 at 22:40
  • It was written by an Englishman. Apr 3, 2019 at 23:53

4 Answers 4


Normally, there is a definite difference in meaning between "will have done" and "must have done".

"Will have done" is the future perfect tense, and it is usually used for discussing conditions in the future. The verb indicates an action that hasn't necessarily happened yet, but it appears from some future point in time to be an event of the past. Example:

By the time you read this note, I will have boarded the train for Kansas City.

"Must have done" is for a belief you have in the present that an event happened in the past. Example:

His toothbrush is gone, and he left this note. He must have gone back to his wife in Kansas City.

However, the quote in your question does not seem to be a normal use of "will have been". From what I can tell of the context, I take the speaker's meaning to be synonymous with "... that must have been a very deliberate decision," or "... that would have been a very deliberate decision."

He is speculating in the present about a past matter that he did not witness.

So why did he use the future perfect tense?

Perhaps because the speaker is imagining in his head a time in the future when everything will be made clear, and at that time it will be known that the decision was very deliberate.

However, these modal verbs (could, can, would, shall, should, may, might etc.) are pretty tricky. People sometimes use them not for their literal meaning, but to express formality ("I should like to ..."), politeness ("could you perhaps ...?"), or equivocation ("May we suppose ...?"), and their uses sometimes differ among different varieties of English.

So it may be that in this gentleman's brand of English, he feels "that will have been" is more formal, or more appropriate to such a learned topic, than "that must have been".

Whatever his reasoning, those two expressions do not usually have the same meaning, but in the context, it really seems he meant the latter.


The construction will have done, where done may be any past participle, known as the future perfect, is usually used to denote past-in-future, that at the time being referred to a certain thing will already have come to pass. It is the auxiliary have forming the perfect, with the addition of the auxiliary will forming the future.

The construction must have done is imply the present perfect (auxiliary have with the past participle) with the modal auxiliary must, indicating certainty, compulsion, or high probability. Put together, that means that the speaker is asserting the likelihood of something happening in the past. "She must have killed him" is a speaker indicating that they consider it certain, or at least highly likely, that she killed him.

Thus, the two are completely different. Except in a case like your example.

Where will have done is used for an event that is actually in the past, it represents a confident supposition. The person is supposing that it was done. This puts it very close in meaning to must have done, though there is a difference in nuance. Will have done tends to convey the idea that this is a logical, reasoned out conclusion, while must have done can refer to a gut feeling.

  • Does the following example fall within the same rule ('will have done' is used for an event that is actually in the past): "Chapter 7: The CPR provides for further rules in other circumstances where the parties have prepared costs budgets in accordance with 3E PD (budget of anticipated costs). You will have seen this considered in more detail in Chapter 4."? Apr 12, 2019 at 18:27
  • @ObliviouslyIgnorant yes.
    – SamBC
    Apr 12, 2019 at 18:47

This question shows why it is so unhelpful to pretend that English has a future (and a future perfect) tense.

Modals (such as can, must, and should) generally have two meanings: a more common deontic meaning, about how the world is, or should, may, must, or will be; and another epistemic meaning, about our knowledge of how events or the world. The latter often has the force of something like "I conclude that ... " or "I expect that..."

"Must" is usually deontic ("You must go and get it!") but sometimes epistemic ("Surely he must see it!"). "Must have" is almost always epistemic, because its past-ness is inconsistent with the particular force of its deontic meaning. Its meaning is something like "I'm sure that ... "

All this applies equally to the modal will (which is syntactically identical with all the other modals). Its deontic meaning is often simple futurity; but can be other things, eg habitual in the present ("Every day, when he gets home, he will sit down and watch the news").

"Will" is rarely epistemic in the present, but in the past (with "have") it can be. Epistemic "will have" means something like "I conclude that ... probably". It is less sure than epistemic "must have".


Given the extent to which the rules of procedure otherwise mirror the CPR, that will have been a very deliberate decision.

The use of will suggests that the writer is confident in assuming that "that"--the decision for the Court Rules not to incorporate the costs management additions to CPR 3--was a very deliberate decision. Although that assumption of course is based in part on "the extent to which the rules of procedure otherwise mirror the CPR", it's not solely based on that. That's why it's called 'assumption' as opposed to 'deduction'.

The use of must, on the other hand, would suggest that the writer is confident in deducing the same from "the extent to which the rules of procedure otherwise mirror the CPR".

Although the writer is confident in either case, I would say 'assumption' is more subjective and less logical than 'deduction'.

So, if the writer wanted "that being a very deliberate decision" to be a logical conclusion from "the extent to which the rules of procedure otherwise mirror the CPR", he would probably have gone with 'must'. But for some reason, he didn't want that. He merely wanted "that being a very deliberate decision" to be a confident assumption based only in part on "the extent to which the rules of procedure otherwise mirror the CPR".

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