Si vuelve el granizo, nos quedamos en casa.

Because after si in a real or possible condition, Spanish normally uses the present indicative:

Si vuelve el granizo, nos quedamos en casa.

This is the standard pattern for an open future condition in Spanish, even though English often uses present + future:

• If hail comes back, we’ll stay home.

So:

• si vuelve = correct
• si volverá = not used here
• si vuelva = not used here either

If the situation were more hypothetical or unlikely, Spanish would change the structure completely:

• Si volviera el granizo, nos quedaríamos en casa.

That means something like If hail came back / were to come back, we’d stay home.

Spanish often uses the present tense to talk about the future, especially in everyday speech when the result feels natural, planned, or immediate.

So nos quedamos en casa can mean:

• we stay home
• or, in context, we’ll stay home

A more explicitly future version would be:

• Si vuelve el granizo, nos quedaremos en casa.

Both are possible. The version with nos quedamos sounds very natural and conversational.

Also, the present can sometimes sound a little more like a regular response or a decided plan.

Volver usually means to return or to come back.

Here, vuelve el granizo means that the hail comes back, returns, or starts again. It refers to the weather phenomenon happening again.

So the idea is:

• If the hail comes back
• If it starts hailing again

A very natural alternative is:

• Si vuelve a granizar, nos quedamos en casa.

That uses the verb granizar = to hail.

In Spanish, weather phenomena are often named with the definite article:

• la lluvia = rain
• el viento = wind
• el granizo = hail

English often leaves the article out, but Spanish commonly includes it.

So el granizo is not strange here; it is the normal way to talk about hail as a phenomenon.

Also, granizo is a masculine singular noun, so it takes el.

Because quedarse and quedar are not the same.

• quedarse = to stay, remain
• quedar = to arrange, to meet, to be left, to suit, depending on context

So:

• nos quedamos en casa = we stay at home

But quedamos en casa would usually not mean we stay at home. It could sound more like we arrange something at home or be incomplete/unclear.

The pronoun nos is part of the verb quedarse.

En casa is the normal idiomatic expression for at home.

So:

• Nos quedamos en casa = We stay home / We stay at home

If you say en la casa, you are usually talking about a specific house as a building:

• Estoy en la casa de Ana = I’m in Ana’s house
• Se quedó en la casa = He stayed in the house

So en casa is the natural choice when you mean at home in a general personal sense.

It can be understood either way, depending on context.

It can mean a future plan or response:

• If hail comes back, we’ll stay home.

But with present tense in both clauses, it can also sound like a general rule or repeated reaction:

• If hail comes back, we stay home.
• Whenever that happens, we stay home.

If you want to make the future meaning more explicit, you can say:

• Si vuelve el granizo, nos quedaremos en casa.

That version points more clearly to the future.

Yes — and many learners will hear that version very often.

• Si vuelve a granizar, nos quedamos en casa.

This means If it starts hailing again, we stay home / we’ll stay home.

The difference is mainly in how the idea is expressed:

• si vuelve el granizo = if the hail comes back
• si vuelve a granizar = if it hails again / starts hailing again

Both are understandable. The version with granizar is often a bit more direct and weather-like.

You would normally use:

• Si volviera el granizo, nos quedaríamos en casa.

This is the classic hypothetical pattern:

• si + imperfect subjunctive
• conditional in the main clause

So:

• Si vuelve el granizo, nos quedamos en casa.
  = real/possible situation

• Si volviera el granizo, nos quedaríamos en casa.
  = more hypothetical/unlikely situation

This is one of the biggest condition patterns in Spanish.

Yes. You can also say:

• Nos quedamos en casa si vuelve el granizo.

The meaning stays the same.

The version starting with si puts the condition first, which is very common. It highlights the if part.

When the si clause comes first, a comma is commonly used:

• Si vuelve el granizo, nos quedamos en casa.

When the main clause comes first, Spanish usually does not use a comma:

• Nos quedamos en casa si vuelve el granizo.