Si deja de caer granizo, iremos al parque para ver si todavía se ve el arcoíris.

Because after si for a real or likely future condition, Spanish normally uses the present indicative, not the subjunctive.

So:

• Si deja de caer granizo, iremos... = a real possibility
• Si dejara / dejase de caer granizo, iríamos... = more hypothetical or less likely

English often uses if it stops, and Spanish matches that with the indicative: si deja.

Because the sentence is talking about what will happen if the condition is met.

The pattern here is very common:

• Si + present indicative, future

So:

• Si deja de caer granizo, iremos al parque.

Spanish can sometimes use the present for near-future meaning in everyday speech:

• Si deja de caer granizo, vamos al parque.

But iremos is very clear and standard.

Dejar de + infinitive means to stop doing something.

Here:

• dejar de caer = to stop falling
• deja de caer granizo = hail stops falling

It is a very useful structure:

• dejar de llover = to stop raining
• dejar de hablar = to stop talking
• dejar de fumar = to quit smoking / stop smoking

Spanish can express this idea in more than one natural way.

This sentence uses:

• caer granizo = literally, hail falls

You could also hear:

• si deja de granizar
• si para de granizar

All are understandable.
Dejar de caer granizo sounds descriptive and natural, especially if you want to focus on the hailstones physically falling.

Because in weather expressions, Spanish often uses the noun without an article when speaking generally.

So:

• cae granizo
• cae nieve
• cae ceniza

Here granizo is being treated like a general weather substance, not a specific batch of hail.

If you were talking about specific hail, you might use the article:

• El granizo que cayó ayer dañó los coches.

Because a + el contracts to al.

So:

• ir a el parque → ir al parque

This contraction is required in standard Spanish, except when El is part of a proper name:

• Voy a El Escorial.

Here, parque is just a common noun, so it must be:

• al parque

No. The second si does not mean a condition here. It means whether / if in an indirect question.

So:

• Si deja de caer granizo... = conditional if
• para ver si todavía se ve el arcoíris = to see whether you can still see the rainbow

This is a very common difference in Spanish:

• No sé si viene. = I don’t know if / whether he’s coming.
• Si viene, hablamos. = If he comes, we’ll talk.

Same word, different function.

Todavía means still here.

So it suggests the rainbow may already be fading or may disappear soon:

• ver si todavía se ve = to see if it’s still visible

A very close alternative is aún:

• para ver si aún se ve el arcoíris

In many contexts, todavía and aún are interchangeable, though todavía is often more common in everyday speech.

Because se ve makes the sentence more impersonal or general.

Here, se ve el arcoíris means something like:

• the rainbow can still be seen
• you can still see the rainbow
• the rainbow is still visible

It is not saying specifically we see the rainbow. It is expressing whether the rainbow is visible in general.

This se is very common in Spanish for general statements:

• Aquí se come bien. = You eat well here / The food is good here.
• No se ve nada. = You can’t see anything.

Because the thing being seen is el arcoíris, which is singular.

So:

• se ve el arcoíris = singular
• se ven las montañas = plural

Even though English might use a structure like you can see..., Spanish is agreeing the verb with what is visible:

• el arcoíris → ve
• los arcoíris → ven

Because in this sentence it refers to a specific, identifiable thing: the rainbow that has appeared after the hail.

Spanish often uses the definite article where English does too:

• el sol
• la luna
• el arcoíris

Also, arcoíris is normally masculine:

• el arcoíris

A useful extra note: arcoíris usually stays the same in the plural:

• el arcoíris
• los arcoíris

Yes. Its position is somewhat flexible.

The original:

• para ver si todavía se ve el arcoíris

Other possible placements:

• para ver si se ve todavía el arcoíris
• para ver si el arcoíris todavía se ve

The original version sounds very natural and probably the most neutral.
Putting todavía earlier often makes the idea of still feel slightly more prominent.

It sounds completely natural and standard. It is not especially formal.

A more conversational version might be:

• Si deja de caer granizo, vamos al parque a ver si todavía se ve el arcoíris.

Differences:

• vamos instead of iremos = a bit more conversational
• a ver si instead of para ver si = very common in speech

But the original sentence is perfectly normal and idiomatic.