La sopa se ha enfriado, así que voy a recalentarla en el microondas.

Because se ha enfriado focuses on the change of state: the soup has become cold / has cooled down.

• La sopa está fría = the soup is cold.
  This just describes its current condition.
• La sopa se ha enfriado = the soup has cooled down / has gotten cold.
  This suggests it was warm before, but now it isn’t.

So this sentence is not just describing the soup; it is explaining why the speaker is going to heat it again.

Here, se is part of the verb enfriarse, which often means to get cold / to cool down.

Compare:

• enfriar = to cool something
  
  • He enfriado la sopa = I cooled the soup
• enfriarse = to become cool / to cool down
  
  • La sopa se ha enfriado = The soup has cooled down

In this sentence, the soup is not actively doing something to itself in a literal sense. This is a very common Spanish pattern for describing something undergoing a change.

Because after haber, Spanish uses the past participle in its normal invariable form:

• ha enfriado
• han comido
• hemos visto

With haber, the participle does not change for gender or number.

So even though sopa is feminine, you still say:

• La sopa se ha enfriado

not

• La sopa se ha enfriada

In Spain, the present perfect is very commonly used for actions connected to the present moment, especially when the result matters now.

So:

• La sopa se ha enfriado = The soup has cooled down, and that matters now because I’m going to reheat it.

In many parts of Latin America, people might more often say:

• La sopa se enfrió

Both are correct, but se ha enfriado sounds especially natural in Spain when the situation is still relevant now.

Así que means so, therefore, or so then.

It connects the two ideas:

• La sopa se ha enfriado = The soup has cooled down
• así que voy a recalentarla = so I’m going to reheat it

It shows result or consequence.

It is different from:

• porque = because
  This gives a reason.
• entonces = then / so
  Also possible in some contexts, but así que is a very natural connector here.

Both are correct.

Spanish allows object pronouns like la either:

1. attached to the infinitive
   
   • voy a recalentarla
2. before the conjugated verb
   
   • la voy a recalentar

Both mean I’m going to reheat it.

In this sentence, voy a recalentarla is perfectly natural and very common.

Because la refers to la sopa, which is:

• feminine
• singular
• the direct object of recalentar

So:

• la sopa → la

Examples:

• Voy a comer la sopa → Voy a comerla
• Voy a recalentar la sopa → Voy a recalentarla

If it were masculine, you would use lo:

• el café → voy a calentarlo

Voy a + infinitive is very common in everyday Spanish for something you are about to do or have decided to do.

• Voy a recalentarla = I’m going to reheat it

This sounds natural and conversational.

The simple future is also possible:

• La recalentaré

But that often sounds:

• a bit more formal,
• more literary,
• or less immediate in everyday speech.

In normal spoken Spanish, voy a recalentarla is usually the more natural choice.

Yes, recalentar is a normal and common verb meaning to reheat.

You could also say:

• volver a calentarla = to heat it again
• calentarla otra vez = heat it again

But recalentarla is concise and very natural.

So all of these work:

• Voy a recalentarla
• Voy a volver a calentarla
• Voy a calentarla otra vez

Because in Spanish, this expression normally takes the definite article:

• en el microondas = in the microwave

That is simply the standard way to say it.

Spanish often uses articles in places where English does not, or where English uses them differently.

So the normal phrasing is:

• calentar algo en el microondas
• meter algo en el microondas

Even though microondas ends in -s, in this meaning it is usually treated as a singular noun when it means microwave oven.

That is why the sentence says:

• el microondas

not

• los microondas

unless you really mean several microwaves.

So:

• El microondas está roto = The microwave is broken
• Lo puse en el microondas = I put it in the microwave

The accent mark shows where the stress falls:

• a-SÍ

Without the accent, asi would not follow standard Spanish spelling.

So así is simply the correct written form of the word meaning so / like that / thus, depending on context.

In así que, it is part of the fixed connector meaning so.

No. The h in Spanish is silent.

So ha is pronounced basically like a.

In:

• se ha enfriado

the pronunciation flows as if the h were not there.

This is true for Spanish h in general:

• hola
• haber
• hacer

All have a silent h.

Yes, absolutely.

That version would also be natural:

• La sopa está fría, así que voy a recalentarla
  = The soup is cold, so I’m going to reheat it.

The difference is small but useful:

• se ha enfriado = it cooled down / became cold
• está fría = it is cold

The original sentence is a little more dynamic because it highlights the process that happened.