Se o melão estiver maduro, eu vou comer um pedaço no café da manhã.

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

• Se o melão estiver maduro = If the melon is ripe
• This is about a condition that may be true later, not a fact being stated now.

Compare:

• Se o melão está maduro... sounds unnatural here for standard Portuguese.
• Se o melão estiver maduro... is the normal choice for a future condition.

This is a very common pattern:

• Se chover, eu fico em casa. = If it rains, I’ll stay home.
• Se ele chegar cedo, saímos. = If he arrives early, we’ll leave.

Estiver is the future subjunctive form of the verb estar.

The verb estar is irregular, so its future subjunctive forms are:

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

In your sentence, o melão is third-person singular, so we use estiver.

In Brazilian Portuguese, ir + infinitive is extremely common for talking about the future.

So:

• eu vou comer = I’m going to eat / I will eat

This often sounds more natural in everyday spoken Brazilian Portuguese than the simple future:

• eu comerei

Both are grammatically correct, but vou comer is much more common in casual speech.

Yes. You could say:

• Se o melão estiver maduro, eu comerei um pedaço no café da manhã.

This is correct, but it sounds more formal or literary. In normal spoken Brazilian Portuguese, eu vou comer is usually more natural.

Portuguese uses definite articles more often than English does.

• o melão = the melon

In this sentence, it sounds like a specific melon is being referred to, perhaps one already known in the context.

If you said:

• Se melão estiver maduro...

that would be ungrammatical here.

You need the article with this noun in this structure:

• Se o melão estiver maduro...

Maduro means ripe here.

It agrees in gender and number with melão, which is a masculine singular noun:

• o melão maduro = the ripe melon

If the noun were feminine, you would use madura:

• a banana madura = the ripe banana

If plural:

• os melões maduros
• as bananas maduras

Um pedaço means a piece or a chunk.

So:

• comer um pedaço = to eat a piece

It is a very natural way to talk about eating part of something like fruit, cake, bread, and so on.

Examples:

• um pedaço de bolo = a piece of cake
• um pedaço de melão = a piece of melon

In your sentence, de melão is understood from the context, so it does not need to be repeated.

No is a contraction of em + o:

• em o → no

Here it means at / for breakfast, depending on how you translate it naturally.

• no café da manhã literally has the idea of in/at the breakfast time/meal

This is the normal Portuguese structure.

Examples:

• Eu como pão no café da manhã. = I eat bread for breakfast.
• Ela bebe café no café da manhã. = She drinks coffee at breakfast.

Yes. Portuguese often drops subject pronouns when the meaning is clear from the verb.

So this is also natural:

• Se o melão estiver maduro, vou comer um pedaço no café da manhã.

Because vou clearly shows first person singular, eu is not necessary.

Including eu can add clarity, emphasis, or contrast, but it is not required.

Yes, somewhat. The original sentence is completely natural, but Portuguese allows some variation.

For example:

• Se o melão estiver maduro, eu vou comer um pedaço no café da manhã.
• Eu vou comer um pedaço no café da manhã se o melão estiver maduro.

Both are correct.

Putting the se clause first often emphasizes the condition. Putting it second can sound slightly more conversational in some contexts.

Melão is pronounced roughly like meh-LOWN, with stress on the last syllable.

A few things to notice:

• ão is a nasal sound, very common in Portuguese.
• The final sound is not exactly like English ow; it is nasalized.
• The written accent mark in melão shows the stressed syllable.

So the stress is:

• me-LÃO

The comma separates the conditional clause from the main clause.

• Se o melão estiver maduro, = if the melon is ripe
• eu vou comer um pedaço no café da manhã. = I’m going to eat a piece for breakfast

When the if clause comes first, a comma is standard and natural. If the clause comes second, the comma is usually not used:

• Eu vou comer um pedaço no café da manhã se o melão estiver maduro.

Not with the same meaning.

• Se = if
• Quando = when

So:

• Se o melão estiver maduro... = If the melon is ripe...
• Quando o melão estiver maduro... = When the melon is ripe...

The second version sounds more certain, as if the melon is expected to become ripe. The first one leaves it as a possibility.

Both take the future subjunctive here:

• se estiver
• quando estiver

Yes, it is natural. A Brazilian could absolutely say it this way.

A few common variations would also sound natural:

• Se o melão estiver maduro, vou comer um pedaço no café da manhã.
• Se o melão estiver maduro, eu como um pedaço no café da manhã.
  This can also express a future plan in context.
• Se o melão estiver maduro, vou comer um pedaço amanhã no café da manhã.
  if the speaker wants to specify tomorrow

So the original sentence is perfectly good, clear, and idiomatic.