Se a sopa estiver muito quente, vou comer depois.

Because after se meaning if, Portuguese often uses the future subjunctive when talking about a possible future situation.

So:

• Se a sopa está muito quente... sounds like you are talking about a present fact or a more general situation.
• Se a sopa estiver muito quente... means if the soup is hot / if it turns out to be hot at that future moment.

This is the natural choice in Portuguese for a sentence like this.

Estiver is the future subjunctive form of estar.

The future subjunctive is very common in Portuguese after words like:

• se = if
• quando = when
• assim que = as soon as
• logo que = as soon as

Examples:

• Se eu tiver tempo, vou. = If I have time, I’ll go.
• Quando ele chegar, avisamos. = When he arrives, we’ll let you know.

So in your sentence, estiver is not a random irregular form: it is the correct future subjunctive of estar.

Because estar is used for a temporary condition, and the soup being hot is temporary.

• estar quente = to be hot right now / at the moment
• ser quente = to be hot by nature, as a characteristic

For food, drink, weather, people’s states, and many temporary conditions, Portuguese usually uses estar.

So:

• A sopa está quente. = The soup is hot.
• O café está frio. = The coffee is cold.

Vou comer is the near future construction: ir + infinitive.

It is extremely common in Brazilian Portuguese and often sounds more natural in everyday speech than the simple future.

So both can mean I will eat:

• vou comer
• comerei

But in Brazil, vou comer is much more common in normal conversation.

Portuguese often leaves out subject pronouns when the verb already makes the subject clear.

Here, vou clearly means I go / I’m going to, so eu is unnecessary.

You could say:

• Se a sopa estiver muito quente, eu vou comer depois.

But it is usually more natural to omit eu unless you want emphasis or contrast.

For example:

• Eu vou comer depois, mas ele vai comer agora.

There, eu helps create contrast.

That a is the feminine singular definite article, meaning the.

So:

• a sopa = the soup

Portuguese uses definite articles more often than English does. In this sentence, it refers to a specific soup already understood in the situation.

Some adjectives change form for gender, but some do not.

Quente is an adjective with the same form for masculine and feminine singular:

• o café quente
• a sopa quente

It only changes for number:

• o café quente
• os cafés quentes
• a sopa quente
• as sopas quentes

So quente is already the correct form with sopa.

Literally, muito quente means very hot.

But in context, especially with the second part vou comer depois, it strongly suggests too hot to eat now.

So the literal meaning is:

• very hot

But the natural understood meaning may be:

• too hot

Portuguese often lets the context carry that extra idea instead of using demais or a longer phrase.

Because the object is understood from the context.

In English, you might naturally say I’ll eat it later. In Portuguese, it is very common to omit the object if it is obvious.

So:

• Se a sopa estiver muito quente, vou comer depois.

naturally means:

• If the soup is too hot, I’ll eat it later.

The listener already knows that comer refers to the soup.

Depois means afterwards, later, or then later, depending on context.

In this sentence, vou comer depois means I’ll eat later.

Yes, you could also say mais tarde:

• Se a sopa estiver muito quente, vou comer mais tarde.

That is also natural.
A small difference in feeling:

• depois = later / afterwards
• mais tarde = later on

Both work well here.

It is standard to use a comma when the if-clause comes first.

So this is the normal punctuation:

• Se a sopa estiver muito quente, vou comer depois.

If you reverse the order, the comma is usually not needed:

• Vou comer depois se a sopa estiver muito quente.

Both are correct, but the version with the condition first is very common and clear.