*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:
- *Tomorrow is rain. (ungrammatical, uses a noun)
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:
- It rains a lot in London.
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:
- It'll rain tomorrow. (It will ...)
[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:
- If A, B.
- When A, B.
- After A, B.
We can also make these sentences the other way:
- B if A.
- B when A.
- B after A.
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:
- When I see you tomorrow, I will buy you dinner.
- After the concert finishes, I will go home.
- If you're coming tomorrow, we will have a great time.
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:
- If it's raining tomorrow, I won't go to the park.
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:
- If + [It will rain tomorrow] = I won't go to the park
... 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:
- If it rains, I won't go to the park.
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:
- If it is rains ..... (ungrammatical. It uses BE, but doesn't use -ing)
Hope this is helpful!