1, If you have not received the letter by February 18, please let us know by email.
2, If you do not receive the letter by February 18, please let us know by email.
Both 1 and 2 are talking about the things that may happen in a few weeks later, and I have seen that 1 and 2 are adopted on some universities's web sites.
Does this mean that 1 and 2 are equivalent?
If not, what are their differences then?
Best Answer
We can say:
If the swelling does not go down by tomorrow evening, take two more of these pills.
or
If the swelling has not gone down by tomorrow evening, take two more of these pills.
If there is a nuanced difference between them, 99.9% of native speakers won't be able to tell you what that difference might be.
We can also say:
After a week, the swelling will have gone down enough that you can put your full weight on your foot again.
That's the future perfect.