My question is about the following two sentences:
The dictionary of Japanese grammar tells me that sentence (1) is wrong, because we can never tell if it will rain tomorrow (this sentence is from the book).
On the other hand, a native speaker gives sentence (2) as an example of correct usage of なら.
On the surface, these sources seem to contradict each other. What am I missing?
I don’t think anything’s wrong with sentence 1 - なら can be used to set a condition for what follows, and that’s exactly what you’re doing, no?
Plus, I can’t help but wonder, if you need to be absolutely sure of what comes before, what’s even the point of a conditional? 
If you already know it’s going to rain tomorrow, I’d say using から or ので is way more logical than using なら.
EDIT: actually, Tofugu even describes the exact opposite:
you cannot use なら if the condition you are describing will definitely happen
The dictionary of JP grammar doesn’t state that you need to be sure of what will happen. In fact, they also list the incorrect usage you quote from the Tofugu article.
They say it would be nonsensical in this case to suppose the truth of “it will rain tomorrow”. Meaning, to suppose that before tomorrow comes.
I think what may be happening here is the sentence is ambiguous and they assume one interpretation.
Sure, you can’t be sure if it’ll rain tomorrow, so “if it rains tomorrow, I will cancel the game now” is a bit nonsensical. But that has nothing to do with なら, that goes for any conditional. You can’t act on what you don’t know.
But the way I read it, that says “if it turns out to rain tomorrow, we will cancel the game [also tomorrow]”, which is fine and in fact is exactly the intended use of なら as I see it.
Other than that, I really wouldn’t know what they’re trying to say.
This is a good point, about whether canceling the game is supposed to happen now versus tomorrow. I was thinking along the same lines, but they translate it in the book as “there will probably be no game”.
On further thought, I guess they could still mean like, “there will probably be no game TODAY”… which would be nonsensical in English too, and would solve my confusion.
I’m definitely not fluent enough (especially about producing) to be able to answer questions of correctness,
but I looked in the Dictionary of Basic Japanese Grammar and I think it’s saying that:
明日雨が降るなら
is saying “if it is true that it will rain tomorrow”
vs.
明日、雨なら
“tomorrow, if it rains”
So it’s the same way in English it would be odd to say “if it will rain tomorrow, the game will be canceled” as opposed to “if it rains tomorrow, the game will be canceled.” I think it’s just a subtle distinction about what exactly なら is supposing. (the kind of subtle distinction that a grammar dictionary would try to dig into).
we can verify the state “is it raining” tomorrow, but we can’t verify the state “it will rain tomorrow.” Is perhaps where their wording was going…
Nah that’s not it. The first sentence is definitely weird with the assumption that the match is tomorrow and would be rained out.
なら is this case is like assuming, in the present moment, that something is going to happen then something something. So it’s weird to use it for something in the future like the shiai in this case.
If your friend said something about how it was going to rain tomorrow, and you responded 明日雨が降るならI’ll go shopping today. I would say that’s natural and fine. But just saying their sentence as is to mean that there will probably be no game tomorrow if it rains feels wrong.
I’m a little confused, what’s the distinction between them then? Isn’t the second sentence saying essentially the same thing but without 降る?
It appears you are not the first person to ask this question
Edit: 2nd explanation as per StackExchange:
For the record, this fits with what Maggie-Sensei says about the first sentence sounding unnatural due to the でしょう at the end.
Like I said, it’s weird using it to speculate the shiai cancellation in this case. Using it for things you’ll do is fine. So I guess you could say that while I agree with the fact that sentence one is weird, I don’t agree with their explanation of why. In facf, it’s 100% bullshit. But it’s weird for other reasons.
This is why 〜たら is the best. You basically don’t have to think about it, since it’s so versatile.
Okay, so just to make sure I understand: if, hypothetically speaking, I were the one deciding whether there’s gonna be a game tomorrow and I’m saying 明日は雨が降るなら、試合はないでしょう, that’s fine because I’m the one making that call and I’m saying right now if I see rain tomorrow, game’s off, but if I’m not the one making that call and I’m just speculating it’s kind of weird to say that (but not so much because of なら)?
Yes if you’re calling off the game you can use it.
Just to check, I googled it, and sure enough people call off the game using it. Probably wouldn’t use deshou doe.
But if you’re speculating, yeah.
Alternatively, if you’re just saying there’s never any games when it rains, you can use it because you’re stating the game cancellation
My Japanese grammar book has some explanations about なら that I think are quite on point, so I’ll just share them.
～ならはは「～ば、～たら、～と」と異なる性質を持っています。「～ば、～たら、～と」では前件と後件の時間関係は必ず前件→後件ですが、「～なら」では必ずしもそうでなくてもかまわないのです。例えば、(…)
北海道へ行く（の）なら彼に連絡したほうがいいよ。
北海道へ行ったら彼に連絡したほうがいいよ。
While the first allows you to contact him even before going to Hokkaido (as long as you’ve decided you are going), the second necessarily implies: 1. Go to Hokkaido, 2. Contacting him.
The above clearly contradicts 「明日、雨が降るなら」, since you can’t “be sure” it will rain to allow for a previously acknowledged “if it will rain tomorrow” as @rodan nicely translated.
Further on the book:
「~なら」には対話的な性格が強く、相手が言ったことを正しいと認めた場合にどうなるかを述べるという用法が中心的です。例えば、(…)
A：今東京へ行くんだ。
B：東京へ行くのなら、この本を買ってきて。
This goes exactly with what @Vanilla said, “If your friend said something about how it was going to rain tomorrow”, because in that case someone took the “burden” of deciding the future from you and affirmed “it’s going to rain tomorrow!”, so now you can actually say “Of, if that is the case, I will 〇〇”
Yeah okay, that’d be kinda weird, as if you’re not sure what you’re gonna do
Thanks! That does clear it up, I was more than a little confused, in case that wasn’t obvious yet 
Right, because one will always lead to the other, similarly to how と would work in stating a logical/unavoidable consequence, right?
That’s easy, just use たら wherever! Makes conditionals 100x easier
Grammar: “always use たら except when you shouldn’t”
(I vaguely remember hearing something like that once, but I don’t remember where.)
I think most people go through a phase where they just say screw it and always use tara.
Also, I think the chart isnt completely right. Nara should be fine to use for the kaeru one and would just be slightly different in meaning.
EDIT: nvm the chart says its ok and my eyes just dont work.
The Wasabi article on conditionals seems to take that position.
