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For some reason different Grammar books show a lot of examples of Future Perfect with such adverbs of time (by tomorrow, by next year, by 2050). It may give the impression that you only use such adverbs with the Future Perfect verbs.

"By tomorrow, he will have left Paris." Future Perfect.

BUT: "Tomorrow, he will leave Paris." Future Simple.

or

"By 2050, scientists will have found the cure for cancer." Future Perfect.

BUT: "In 2050, scientists will find the cure for cancer." Future Simple.

In the wild, I've come across examples such as:

"Do you think you can deliver it by tonight?" Present Simple.

"I will have a lot of money by tomorrow" Future Simple.

So what's the matter?

I'll even go so far as to ask: Is it grammatical to say "I will do my homework by tomorrow" or "I will leave Paris by tomorrow" instead of "I will have done my homework by tomorrow" or "I will have left Paris by tomorrow". If it is, what's the point of using Future Perfect with such adverbs of time?

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Yes, all those sentences are grammatical.

By tomorrow, he will have left Paris. When tomorrow comes, he will no longer be in Paris. It doesn't say exactly when he will leave.

I will leave Paris by tomorrow. I promise/intend to leave tomorrow or earlier.

I will have left Paris by tomorrow. I confidently predict that when tomorrow comes, I will no longer be in Paris. It doesn't say exactly when I will leave.

I will do my homework by tomorrow. I promise to get it finished by tomorrow.

I will have done my homework by tomorrow. I confidently predict that it will be finished when tomorrow comes.

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  • Scientists will find the cure by 2050. Scientists promise/intend to find the cure in 2050 or earlier? "Scientists promised that they would find the cure by 2050" Does it sound good? – Rusletov May 12 at 19:58
  • Can I say that "By 2050, the cure will be found" says only about the state of the cure being found by that time. Whereas "By 2050, the cure will have been found" implies both the state of it being found and the action done by someone (found by scientists). – Rusletov May 12 at 20:01
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    To say that a cure will have been found by 2050 is to predict that, at that date, the cure will already exist, whether it was discovered in 2030, 2040 or whenever. To say scientists will find a cure by 2050 refers to their present hopes of finding one in the next 30 years. – Kate Bunting May 13 at 6:59

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