Which one is correct?:
- if you (will not come) by tomorrow ,let me know
- 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?
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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.