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I don't think there is any difference, either in choice of grammar or meaning, between, e.g.,

'Everything will be done by Tuesday.'
'Everything will have been done by Tuesday.'

except for the second one being more formal/grammatically rigorous. Is there?

  • The 3 answers provided so far suggest that there IS a difference. However, there is no consensus as to what the difference is, so I would appreciate more people weighing in to tip the balance. For my part, I could identify no difference, believing the two versions to be similar to 'On (not when) arriving at ...' and 'On having arrived at ...', respectively. I accepted John's answer because he offered an insight that revealed a whole new angle for me. Ariane's answer has also been very helpful, though in a more familiar context for me. – Alexander Demidov Jan 27 '18 at 11:02
  • At least one answer to this question reposted elsewhere endorses my reading. – Alexander Demidov Jan 27 '18 at 16:08
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'Everything will be done by Tuesday.'

This is a future simple tense, which means, 'everything' will be done in the future, with no indication whether you are doing it now(let's say it is sunday at the moment), at least it will be done by Tuesday.

'Everything will have been done by Tuesday.

Means, you are currently doing it now(from sunday to tuesday), clearly, this is future perfect tense. Will+have

  • 'Everything will have been done by Tuesday. Means, you are currently doing it now(from sunday to tuesday). This is an interesting insight; I wasn't aware of that angle. Thank you, John. – Alexander Demidov Jan 26 '18 at 13:41
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There is a difference. This is simply the difference between future simple and future perfect

The difference is from "when" you are stating the fact. Future perfect (will have been done) is you jumping forward to Wednesday and speaking to the work that has definitely been done.

The future simple tense is no more than a vague statement that the work will probably be done sometime in the future with a slight commitment of a day, but not much more than that. I use this tense :)

Future

The future indicates that an action is in the future as referred to from now.

Future perfect

The (indicative) future perfect indicates that an action will have been completed (finished or "perfected") at some point in the future. The main differences are that

  • This is in relation to a point in time
  • This is generally guaranteed to be completed
  • Indicates the state of the event, more specific
  • Speaks as if in the future, next time we speak it will have been done, a promise. The work is now in the past and guaranteed to have been completed.
  • Quite an informative answer. Thank you, Ariane. I do feel, though, that the introduction of the preposition 'by' effectively equates what is 'no more than a vague statement that the work will probably be done sometime in the future with a slight commitment of a day' with an iron-clad guarantee that it most certainly will. – Alexander Demidov Jan 26 '18 at 14:05
  • By Tuesday is yes, quite vague, implying "any time up to end of day" Tuesday. However, the addition of "will have been done" then injects a certainty. I promise that it will have been done, so whilst the exact date and time is vague, the implication is that it will certainly be done, much more so that future simple. – Ariane Kh Anderson Jan 26 '18 at 16:42
  • This makes perfect sense to me now. Thank you, Ariane, for your clarification. – Alexander Demidov Jan 27 '18 at 7:12

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