Learn English – Usage of “both” vs. “either”

The reason this sentence is confusing is the ambiguous "it is". Depending on what the listener believes you are referring to, the sentence may take on different meanings.

"The forecast for today is 'rain' again."

"If it (the forecast) is 'rain' tomorrow, we won't go to the park."

In this case, "rain" is a quote about the weather. Since it is a quote, it does not have a tense and is correct.

"(The sky) is raining."

"If it (the sky) rains tomorrow, we won't go to the park."

Now we are talking about an action that the sky is taking - it has rained, it will rain, or it is raining. Since we are talking about a state that the sky will be in for some time, we say "it rains".

The reason that the first sentence ("if it is rain") sounds wrong to a native speaker is that they are assuming rain is a verb associated with "it" and thus should be conjugated as such.

Don't (do not) is the plural form, doesn't (does not) is the singular. She does, he doesn't, they do, they don't.

And, especially relevant to the question, someone does and someone doesn't.