Is it correct to state that "by" is used with Future Perfect tense in affirmative sentences, and "until" / "till" are interchangeable and used only in negative statements? And what about interrogative ones ("by" similar to affirmative)?

I'm still not sure is there any real difference, though I've tried to clear it up.

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    No, it's not true. There's nothing unusual about, say, I'll be here until 5 o'clock, and I see no "negation" there. Of course, I could also say I won't be there until 5 o'clock, and I could replace until with by in both cases to "reverse" the meaning. Commented Apr 13, 2017 at 17:41
  • @FumbleFingers, but what about Future Perfect tense? Commented Apr 15, 2017 at 10:49
  • What about it? By the time you can understand this sentence, you will have understood how it works. Alternatively, Until you can understand this sentence, you will not have understood how it works. In those examples, the semantics make it easier to pair by with a "positive" assertion, and until with a negative one. But until you can understand this example, you will have been limited in your grasp of how English tenses can validly be used. Your by=affirmative, until=negative idea is largely spurious. Commented Apr 16, 2017 at 14:06

1 Answer 1


I won't have finished X until|by... grammatical

I won't have begun it until|by ... grammatical

I will have finished X until... ungrammatical

I will have begun it until ... ungrammatical

You cannot express a completion with a lifespan.

But you can express a process with a lifespan:

He will have eaten it until it made him sick. He cannot control himself with chocolate cake.

Knowing him, he will have played that guitar until his fingers blistered.

He will have waited there until the lights went out.

  • +1 - Is it wrong to say : until his fingers are blistered?
    – Johansson
    Commented Apr 13, 2017 at 18:10
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    I think the present tense is not impossible there, though it sounds marginal to me.
    – TimR
    Commented Apr 13, 2017 at 18:24
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    It seems to me all your final three "process" examples refer to what he has already done - but although strictly speaking we don't know this for sure, that's what we will/would establish with certainty if we were to investigate the matter more thoroughly. This is somewhat different to a "real" future reference such as, say, By midnight tonight he will have waited there for twelve hours (which idiomatically we'd be more likely to express with the continuous ...he will have been waiting). Commented Apr 16, 2017 at 14:14
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    They don't refer to "what he has already done" but to "what he will have done" as of a certain time in the future.
    – TimR
    Commented Apr 16, 2017 at 15:07
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    @Anthony Voronkov: There is no rule which says the present simple is the only tense that can be used with "after", "before", "as soon as", "by the time". Good that you don't think so now, but only used to think so.
    – TimR
    Commented Apr 16, 2017 at 15:12

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