Which one is correct?:

  1. if you (will not come) by tomorrow ,let me know
  2. if you (do not come) by tomorrow ,let me know

I have read posts here about using "will" in the if clause.in this answer

Is using "by tomorrow" affects the answer?

  • 1
    Well, "do not come" isn't very helpful because they've already not attended.
    – Catija
    Jun 8, 2015 at 16:25
  • When do you want to be let know? Today or tomorrow? Jun 8, 2015 at 16:28
  • Can you please explain your reason for changing the question? What do you think that "by" does? It's generally not acceptable to change the question after you have already received answers.
    – Catija
    Jun 8, 2015 at 16:34
  • @catija sorry about that. i didn't write my question correctly the first time. I think using "by tomorrow" may have implication on the tense Jun 8, 2015 at 16:34
  • It's just that "If you will not come by tomorrow" can mean something completely different than "if you will not come tomorrow". So now you need to explain how you're using "by".
    – Catija
    Jun 8, 2015 at 16:35

3 Answers 3


If you want to use "will" I would generally recommend something like:

If you will not be coming tomorrow, please let me know.
If you will not attend (the event) tomorrow, please let me know.

Alternately, I'd argue the best word for this exact phrase would be to use "can". Can is great because it implies that they're unable to attend due to some other reason whereas "will" can imply they simply refuse to attend:

If you can not come tomorrow, please let me know.

I would not recommend using "do" in this situation because it is pointless.

If you do not come tomorrow, please let me know.

Let's say today is Monday, so tomorrow is Tuesday. You're implying that, should the person not come on Tuesday, they should let you know about it after the fact, possibly on Wednesday.


The two mean different things.

  • "If you do not come tomorrow, let me know" means that you are expecting to be informed after the event - i.e. it's OK to let me know the day after tomorrow that you did not show up.
  • "If you will not come tomorrow, let me know" does imply that you expect to be informed before the event. I think a native speaker is more likely to say "If you are not going to come" or "If you are not coming", but I can't see anything grammatically wrong with this.

The first example sounds a little awkward to my ears. I would probably say "Let me know if you aren't going to come tomorrow." The second example sounds OK to me.

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