Se a melancia estiver madura, eu vou levar um pedaço para o almoço de domingo.

Because after se when you are talking about a possible future situation, Portuguese usually uses the future subjunctive.

• Se a melancia estiver madura... = If the watermelon is ripe... / If the watermelon turns out to be ripe...
• Se a melancia está madura... sounds wrong in this context.

A useful pattern is:

• Se + future subjunctive, main clause in the future / present / imperative

Examples:

• Se chover, eu não vou. = If it rains, I’m not going.
• Se ele chegar cedo, nós saímos. = If he arrives early, we leave.

So estiver is the future subjunctive form of estar.

Estiver is one of the subjunctive forms of estar. It may look unfamiliar because irregular verbs often change shape in the subjunctive.

For estar, the future subjunctive forms are:

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

In this sentence:

• Se a melancia estiver madura...

the subject is a melancia, which is third person singular, so estiver is the correct form.

Because adjectives in Portuguese usually agree with the noun they describe.

• a melancia is feminine singular
• so the adjective must also be feminine singular: madura

Compare:

• o abacate maduro = the ripe avocado
• a banana madura = the ripe banana
• as frutas maduras = the ripe fruits

Portuguese uses definite articles more often than English does.

• a melancia literally = the watermelon

Even when English might just say watermelon, Portuguese often prefers a melancia when referring to a specific one or the one already understood from context.

So Se a melancia estiver madura sounds natural as If the watermelon is ripe.

Both are possible, but eu vou levar is much more common in everyday Brazilian Portuguese.

• eu vou levar = I’m going to take / I’ll take
• levarei = I will take

The simple future (levarei) is grammatical, but in conversation it can sound more formal, written, or less natural in many everyday situations.

So:

• Se a melancia estiver madura, eu vou levar... sounds very natural in spoken Brazilian Portuguese.

Yes. Portuguese often allows the subject pronoun to be omitted when the verb already makes the subject clear.

So both are natural:

• Se a melancia estiver madura, eu vou levar um pedaço...
• Se a melancia estiver madura, vou levar um pedaço...

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

This is about point of view.

• levar = to take something away from where you are
• trazer = to bring something toward where the speaker/listener is

In this sentence, the speaker imagines taking the watermelon piece to the Sunday lunch, so levar is the natural choice.

• vou levar um pedaço para o almoço = I’m going to take a piece to the lunch

If the speaker were focusing on bringing it to the place where the listener is, trazer might be possible in another context.

Yes, both are possible, but they are slightly different.

• um pedaço = a piece
• uma fatia = a slice

Pedaço is more general. It can mean any piece or chunk. Fatia suggests a slice, usually something cut in a flatter or more regular shape.

With watermelon, both can work:

• um pedaço de melancia = a piece of watermelon
• uma fatia de melancia = a slice of watermelon

In your sentence, um pedaço is simply a general, natural choice.

Because it is understood from context.

The sentence already mentions a melancia, so um pedaço naturally means a piece of it.

Full version:

• vou levar um pedaço de melancia

But once the watermelon has already been mentioned, Portuguese can leave that out:

• vou levar um pedaço

This works in English too:

• I’ll take a piece
  when everyone knows a piece of what.

It means for Sunday lunch or for the Sunday lunch meal.

Breakdown:

• para = for / to
• o almoço = the lunch
• de domingo = of Sunday / Sunday

In natural English, we would usually say:

• for Sunday lunch
• to Sunday lunch is less likely here

So the idea is that the speaker plans to take the watermelon for the lunch that happens on Sunday.

Because de domingo means Sunday as a descriptive label, not of the Sunday as a specific one.

• almoço de domingo = Sunday lunch
• jantar de sábado = Saturday dinner

This is similar to how English uses a day name like an adjective.

If you said do domingo, it would sound more like of the Sunday, referring to a very specific Sunday already identified. That is less natural here.

So:

• almoço de domingo = the normal way to say Sunday lunch

It refers to a future possibility.

Even though English says If the watermelon is ripe, Portuguese uses the future subjunctive to show that the speaker is not stating a fact now, but talking about what may be true later.

So the meaning is something like:

• If the watermelon is ripe (when I check / when the time comes), I’ll take a piece...

It is not saying the speaker already knows that it is ripe.

The sentence has two parts:

1. Se a melancia estiver madura

   • conditional clause: If the watermelon is ripe

2. eu vou levar um pedaço para o almoço de domingo

   • result/main clause: I’m going to take a piece for Sunday lunch

So the overall pattern is:

• Se + condition, main result

This is a very common Portuguese structure:

• Se eu tiver tempo, vou te ligar. = If I have time, I’ll call you.
• Se eles puderem, vão participar. = If they can, they’ll participate.

Yes, but it changes the style slightly.

• Se a melancia estiver madura... = If the watermelon is ripe...
• Caso a melancia esteja madura... = In case the watermelon is ripe... / If the watermelon happens to be ripe...

Caso is a bit more formal or more careful in tone. Also, after caso, Portuguese normally uses the present subjunctive:

• caso esteja

So both are grammatical, but se ... estiver is the most common and natural everyday choice here.