I've come across this:

Sometimes, you can use the future perfect tense and the simple future tense interchangeably. In these two sentences, there is no real difference in meaning because the word before makes the sequence of events clear:

Linda will leave before you get there. Linda will have left before you get there.

But without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect to show what happened first.

That means: "Mary will leave by the time you get there." is equal to "Mary will have left by the time you get there."

So, is the source correct?

  • Both sources are correct. Linda will leave before you get there is a valid sentence, but if you use 'by the time' it requires the future perfect. (At the moment when you arrive, she will already have left.) May 24, 2021 at 13:25
  • But the second source says that "without prepositions such as before or by the time that make the sequence of events clear, you need to use the future perfect" This means if I use "before" or "by the time" I can use the future simple instead of the future perfect. So the second source contradicts to the first.
    – Let
    May 24, 2021 at 13:39
  • 1
    Yes, OK, the second one is confusing. May 24, 2021 at 13:51
  • 3
    Linda will leave before you get there is OK, but Linda will leave by the time you get there is not (in my opinion). May 24, 2021 at 15:22
  • 2
    by and by the time are not the same thing at all.
    – Lambie
    May 26, 2021 at 18:18

3 Answers 3

  1. will leave (not now. But in the future)
  2. by next Monday (to be finished by Monday at the latest. 1+2= will leave before Monday or will leave before Tuesday.

Future perfect means sth will be 100% finished. 'will have left'=already left by Monday, stressing 100% sth will be done.

Just substitute 'Monday' for 'the time~'.

  • No. A (future or past) perfect verb phrase is choosing to look back at the event or activity from some later time. Obviously that implies that the event or activity is finished at that focus time; but not using a perfect construction does not imply the converse: it simply chooses not to set a temporal focus that looks back on the activity or event.
    – Colin Fine
    May 26, 2021 at 23:21
  • Can you explain more coherently? I can not grab the point.
    – gomadeng
    May 27, 2021 at 6:09
  • Sure. He'll write the report by next Monday and He'll have written the report by next Monday are both completely grammatical, and natural, and refer to the same sequence of events. They do not differ in whether or not he completes writing the report, but only in the temporal viewpoint: whether or not you are looking back from next Monday or not.
    – Colin Fine
    May 27, 2021 at 19:51
  • Thx. Structrure: A will "leave/have left" by B. We need only to analyze "leave vs have left"+will. The OP knows 'will leave'. The focus is 'have left'. This means 100% done already and we know need 'when or by when': temporal viewpoint. Finishing sth 100% raises more importantly 'time concept' than simple future 'will'.
    – gomadeng
    May 28, 2021 at 4:34

When we use the expression "by the time...", we mean that one thing will have already happened before that time arrives. So, in the sentence you would expect there to be one event to mark the time, and then another event which will have happened before that time.

For example:

By the time he arrives we will already have finished dinner.

The expression "by the time..." has to be qualified by a marked event in the future, ie the time when he will arrive, so you use the present tense "he arrives" for this. But then, the second event you expect to have concluded before that time is something that will already have happened, so this requires the future perfect tense, because you are speaking about it happening from the perspective of the time you already established.


From what I've understood so far:

  1. One shall use a perfect construction in the context when they've mentioned some future event or action, and they're mentioning some prior action or state which is an "earlier future" for that future action or state. For example,

"The movie starts at 10. Will you come?" "Let me see... I will have finished my work at 9:30, so I can come no problem"

In a way, I'm imagining the future (the moment when the movie starts; 10 pm) and I confidently predict that at that moment I will be able to say "I have finished my work" or "I finished my work half ah hour ago". I'm very self-assured at that moment. enter image description here

  1. One shall use the future simple construction when they just assert a range of time within which they intend to complete an action:

"Hey, what are your plans for today?" "Well, I don't have many. I think I will finish my work by 9 pm and then I'll be able to relax. Stop asking me questions, I'm already worked up as it is." enter image description here

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