Why is this incorrect?
If it is rain tomorrow, we won't go to the park.
and why should it be this?
If it rains tomorrow, we won't go to the park.
[edit]Now I added 'Why' in hope it's understood as needing help. Well, I didn't know the sentence is so obviously incorrect, and I still don't understand why. Please help me.
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.
*If it is rain tomorrow, we won't go to the park. (ungrammatical)
If it rains tomorrow, we won't go to the park.
Example (1) is ungrammatical when RAIN is a verb. Usually, when we talk about the weather tomorrow, we use a verb, NOT a noun. We don't say:
Talking about the present
The verb RAIN usually uses the word it as a subject:
In this example we see the Present Continuous is raining. This is because the raining action is happening NOW. Notice that we use is and an -ing form of the verb together.
If we want to say that it rains generally, usually, or often, we use the Present Simple:
Notice that the verb rain has an 'S' here. Also, there is no verb is in this sentence. We don't use BE (am/is/are) as an extra verb in the Present Simple.
Talking about the future
When we want to say that the weather will be rainy tomorrow, there are different ways to do this. If we saw the weather forecast on the television, then we can say:
If we saw the weather forecast, OR if we just have an idea that it's going to rain, or someone told us, we can say:
[There are some other ways too, but they aren't very important for us now.]
Conditionals (The future and if)
In sentences beginning with if or time prepositions like when, before, after, until the sentence often has two parts, two clauses:
We can also make these sentences the other way:
The important grammar here is that we do not like to use will in the A section. We don't like to use will after words like if, after, when. We usually use a normal present tense of the verb.
We can use will in the B section:
The Original Poster's examples
The original Poster wants to say they won't go to the park in rainy weather. They are using a sentence with if to talk about the future. If they have seen the weather forecast they can say:
Notice that this example uses the present continuous. We need the verb BE ('s) and the -ing form of RAIN.
If the speaker wants to say:
... then they need to change the first sentence, because we don't like to use will after if. We need to use the present simple:
Notice that we have two choices. Sentence (3) uses is raining, the Present Continuous. Sentence (4) uses rains, the Present Simple. We can use "is + -ing" OR we can use just the main verb "rains". But! We cannot mix them. We cannot use the verb BE ('s) and -ing! The following example is wrong:
Hope this is helpful!
As said in the comments, both are fine. However, I would say that the second one:
If it rains tomorrow, we won't go to the park.
sounds much more natural to me. I would be happy with, and would understand
If it is rain tomorrow, we won't go to the park.
but I would always favour the prior (the first one).
((Why is this incorrect?
If it is rain tomorrow, we won't go to the park. and why should it be this?
If it rains tomorrow, we won't go to the park.))
In the sense of "rain" falling from the sky you should say "it rains", where the word rain is a verb. "It is rain tomorrow" is an incorrect sentence; a "day" cannot be "rain"! You should say here "it'll rain tomorrow". However, with conditional "if" you don't use "will" in the same clause. So the correct sentence is "if it rains tomorrow, we won't go out", "if clause" takes the present simple when talking about the possible future.
I don't think it's necessarily "incorrect", the ambiguity comes from the word "it" in this sentence.
"What is the weather forecast for tomorrow?" "It is rain" --> the weather forecast is rain.
It's not regularly used this way though, a "normal" response would be "I've heard it'll be raining tomorrow"
