The number of books in the library ____(increase) by at least fifty percent by the time the renovation finishes next year.

  1. will increase
  2. will have increased

Why are both answers correct?

And when do you use one instead of the other?

Does it have anything to do with asserting how things will play out in the future and confidently predicting something?

  • 2
    At the time the renovation is complete - a date in the future - , the library's stock will be 50% larger than it is now; it will (already) have increased, Will increase doesn't sound jarringly wrong, but it seems more logical to say "will increase over the next year" and "will have increased by next year". May 15 at 13:27

They're both grammatically correct because there's no grammatical problem with either.

It's a subtle distinction. The difference is about what time the words refer to.

"...will have increased" refers to the indicated point in time.

"... will increase" refers to the future in general, so in this case, the period leading up to the indicated point.

I expect in this case the writer means to make a point about the indicated point in time, so "will have increased" would help focus that a bit.

On the other hand, if the writer (say, outside this sentence) is instead focusing on describing other things about the increase in books, and the renovation's completion is just an incidental point of reference, the writer might prefer "will increase".

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