Se o mamão estiver maduro, minha irmã vai comer mamão com uva no café da manhã.

Because Portuguese normally uses se + future subjunctive for a condition that refers to the future and is still uncertain.

Here, the ripeness of the papaya is not being presented as a fact right now; it is a possible future condition. So estiver is the expected form.

• Se o mamão estiver maduro... = if the papaya is ripe / if the papaya turns out to be ripe
• Se o mamão está maduro... sounds unnatural for this standard future-condition meaning

For learners, a very useful pattern is:

• se + future subjunctive
• then a main clause about what will happen

Estiver is the future subjunctive of estar.

Brazilian Portuguese uses the future subjunctive a lot after words like:

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

Examples:

• Se ele vier, eu falo com ele.
• Quando você chegar, me avise.

So in your sentence, estiver fits this very common pattern.

It is an irregular form of estar.

The future subjunctive forms are:

• eu estiver
• você/ele/ela estiver
• nós estivermos
• vocês/eles/elas estiverem

So se o mamão estiver maduro means if the papaya is ripe.

This is worth memorizing because several common verbs are irregular in this tense:

• estar → estiver
• ter → tiver
• fazer → fizer
• dizer → disser
• vir → vier

Both are correct, but vai comer is much more common in everyday Brazilian Portuguese.

• vai comer = near future / natural spoken future
• comerá = simple future, often more formal, written, or less conversational

So:

• Minha irmã vai comer mamão... sounds very normal in speech
• Minha irmã comerá mamão... is grammatically correct, but more formal

A native speaker in Brazil will very often choose ir + infinitive in conversation.

Yes. That sentence is correct.

The difference is mostly style:

• vai comer = more natural and common in spoken Brazilian Portuguese
• comerá = more formal, more literary, or more typical of careful written language

So the original sentence sounds very natural for modern Brazilian Portuguese.

Because the first o mamão refers to a specific papaya.

• Se o mamão estiver maduro... = if the papaya is ripe

Later, comer mamão is a more general food expression, where Portuguese often leaves out the article:

• comer mamão
• comer pão
• beber café

So the second mamão works like the name of the food, not necessarily as a strongly marked specific object.

That said, vai comer o mamão is also possible if you want to emphasize that she will eat that specific papaya.

Because repeating the noun is often more natural in Portuguese than using an object pronoun, especially in everyday Brazilian speech.

A more pronoun-based version would be:

• Se o mamão estiver maduro, minha irmã vai comê-lo com uva no café da manhã.

That is correct, but it sounds more formal or less conversational to many Brazilian speakers.

In natural spoken Brazilian Portuguese, repeating the noun is often the easiest and most natural choice.

Because here uva is being used as a food item in a general way, like an ingredient or accompaniment.

• mamão com uva = papaya with grapes

Portuguese often omits the article in this kind of food expression.

If you said com a uva, it would usually sound more specific, as if you were talking about a particular grape or a particular batch of grapes already known in the conversation.

So com uva is the natural choice here.

No is a contraction of em + o.

• em o → no

This kind of contraction is extremely common in Portuguese.

Other common ones:

• de + o = do
• de + a = da
• em + a = na
• a + o = ao

So:

• no café da manhã = at breakfast / during breakfast

Café da manhã is the normal Portuguese expression for breakfast.

Literally, it comes from coffee of the morning, but as a fixed expression it simply means the morning meal.

So learners should treat it as a set phrase:

• tomar café da manhã = to have breakfast
• no café da manhã = at breakfast

You do not need actual coffee for it to count as café da manhã.

Because in Brazilian Portuguese, possessives can appear with or without the definite article.

Both are possible:

• minha irmã
• a minha irmã

In Brazil, leaving the article out is very common and natural, especially in neutral everyday speech.

So minha irmã here is perfectly normal.

Because adjectives in Portuguese usually agree with the noun in gender and number.

• mamão is masculine singular
• so the adjective is maduro

Examples:

• o mamão maduro
• a banana madura
• os mamões maduros
• as bananas maduras

So maduro is used because it matches mamão.

Yes. You can put the main clause first and the se clause later:

• Minha irmã vai comer mamão com uva no café da manhã se o mamão estiver maduro.

That is also correct.

A useful punctuation rule:

• If the se clause comes first, a comma is normally used.
• If it comes second, the comma is often omitted.

So both versions work; the original just puts the condition first.

Not always, but very often when the condition refers to the future.

In this sentence, yes: se introduces an uncertain future condition, so Portuguese uses the future subjunctive.

But se can appear in other structures too. For example:

• Não sei se ele vem.
  Here se means whether, not if in a conditional sense.

So the key point is not just the word se, but the kind of clause it introduces. In your sentence, it is a future condition, so estiver is the right choice.