If I will show up, I'll let you know at the last minute. Correct?

Not sure whether I'll show up, but if I will, I'll probably let you know at the last minute.

Is it correct or should it be "if I do"? I'm on the fence here because showing up happens after letting them know that I will show up.

So the sequence goes like this: If I decide I'm going to show up, I'll warn them at the last minute before showing up, and then I'll show up.

• Can you find three, or two or any instances where "If I will…" might work? Even "If I do…" can't work in this instance. To work, you need at one more level on contingency… as for instance "If I do plan to…". Do those differences not make sense? Nov 20, 2022 at 17:54

In your sentence, "will" is incorrect.

The only time we can normally use "will" in an "if" clause is when the "if" clause represents the consequence of the main clause, that's to say, the "if" clause itself is conditional upon the main clause. For example:

If it will keep him quiet, then yes, he can watch TV.

Here, the main clause causes the "if" clause to happen. The "if" clause is contingent upon the main clause. If he watches TV, he will be kept quiet. So "will" is OK in the "if" clause.

In your example, the "if" clause, "If I will come", is not conditional upon the main clause, "I'll probably let you know at the last minute". That's to say, letting you know at the last minute will not cause me to come to the event.

I would be more likely to use show up when I was the person expecting someone else to arrive, although Cambridge seems to be the only dictionary that agrees with me. I would say

Not sure whether I can make it, but if I [find I] can, I'll let you know at the last minute.

In your version, I would use if I'm going to rather than if I will. The sequence of events is: I decide after this conversation that it will be possible for me to meet my friends, I let them know that I am planning to come, then I arrive.

• Show up and can make it mean two completely different things to me here. If I’m not sure whether I can make it, I’m saying I predict external factors that may prevent me from going; if I’m not sure whether I’ll show up, there are no external factors, I’m just not sure if I can be bothered. Oct 30, 2022 at 9:18
• @JanusBahsJacquet You Post a perfect example… Do you truly believe "If I will show up" and "If I will can make it" are in any way comparable? Nov 21, 2022 at 23:58

The tense in "if" and "when" clauses is the simple present, (unless you are forming hypothetical second or third conditionals) So you would use "if I do".

I don't find this very "logical" but that is the usual grammar, and there are exceptions.

• This answer (english.stackexchange.com/a/56936/362515) seems to confirm that it should be "but if I will" in my example, since the showing up occurs AFTER the letting them know. Do I get it correctly?
– Let
Oct 29, 2022 at 11:00
• In the given example, I prefer "if I do". As I said, I don't think that is logical. Oct 29, 2022 at 11:10
– Let
Oct 29, 2022 at 11:45
• It’s the subjunctive mood. Oct 30, 2022 at 3:57
• I would probably say "if I'm going to" or "if I decide to" Oct 30, 2022 at 22:02

This sounds like first conditional (unreal, but likely) so the word "will" has kind of an important role here. That, and your excessive use of "will" (and its contraction) are what make your sentence sound so wrong.

Here is a slightly more formal rewrite:

I am not sure whether I will show up, but if I will, I will probably let you know at the last minute.

Ugg. If I were to rewrite the sentence for common use, it would be one of these, in order of preference:

I'm not sure if I'm going, but if I do, I'll probably let you know.

I'm not sure if I'll go, but if I do, I'll probably let you know.

I'm not sure if I'm going, but if I go, I'll probably let you know.

I'm not sure if I'll show up, but if I do, I'll probably let you know.

And, if you're so inclined, "whether" can be substituted for "if" in these sentences to create the next 4 in the series. But regardless, I don't think you'd want to use "will" in the conditional as you have.

My suggestion:

Not sure whether I'll [come/go], but if I do, I'll probably let you know at the last minute.

When you use "show up", the rest of the sentence sounds conditional on your arrival. By changing this to "come" or "go" it is conditional on you making your way to the event. This way your notifying occurs with the conditional, rather than prior to the conditional.

You should say

If I show up, ...

“If I do show up,” suggests I probably will not. “When I show up,” is a promise that I will.

This is the subjunctive mood. Similarly, we say, “If I were there right now,” in the present subjunctive (in standard written English, but we can say if I was informally). We say, “If I had shown up yesterday,” in the past subjunctive.