Se fizer sol, vou levar um chapéu e outra camisa.

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

So:

• Se faz sol would sound wrong for this meaning.
• Se fizer sol = if it’s sunny / if it turns out sunny

This is very common in Portuguese:

• Se eu tiver tempo... = if I have time...
• Se ele puder... = if he can...
• Se chover... = if it rains...

In English, we often use the present after if, but in Portuguese this kind of future idea often requires the future subjunctive.

Fizer is the future subjunctive form of fazer.

Here, the verb fazer is being used in the weather expression fazer sol, which means to be sunny.

A few forms of the future subjunctive of fazer are:

• se eu fizer
• se você fizer
• se ele/ela fizer
• se nós fizermos
• se vocês fizerem
• se eles fizerem

So se fizer sol literally follows the pattern if it does/makes sun, but naturally it means if it’s sunny.

Because fazer sol is the normal everyday weather expression in Portuguese.

Portuguese often uses fazer with weather conditions:

• faz sol = it’s sunny
• faz calor = it’s hot
• faz frio = it’s cold

So even though it may feel strange to an English speaker, fazer sol is the natural expression.

Vou levar is the very common spoken way to talk about the future in Brazilian Portuguese.

It is formed with:

• vou = I go / I’m going
• levar = to take

Together: vou levar = I’m going to take / I’ll take

You could also say levarei, which is the simple future, but in everyday Brazilian Portuguese vou levar is usually more natural and more common in speech.

So:

• vou levar = common, natural, conversational
• levarei = correct, but often more formal or written

Here, levar means to take in the sense of bringing something with you.

So vou levar um chapéu means:

• I’m going to take a hat
• or more naturally in context, I’ll bring a hat

Portuguese uses levar where English may use either take or bring, depending on point of view.

In Portuguese, outro / outra often works without an article.

So these are both natural:

• outra camisa
• uma outra camisa

But outra camisa is very common and simple.

That means the sentence is perfectly natural as:

• um chapéu e outra camisa

The first noun gets the indefinite article um, while outra already gives enough determiner-like information for the second noun.

It can mean either, depending on context.

Most often, outra camisa means:

• another shirt
• an extra shirt
• a different shirt

In this sentence, it probably suggests an extra/change of shirt or a different shirt to wear, depending on the situation.

Portuguese does not always separate those ideas as clearly as English does.

Because outro / outra normally goes before the noun in Portuguese.

So:

• outra camisa = another shirt
• outro livro = another book

That placement is the standard one.

It is the normal punctuation choice, yes.

When a conditional clause comes first, Portuguese usually separates it with a comma:

• Se fizer sol, vou levar um chapéu...

This is similar to English:

• If it’s sunny, I’ll take a hat...

If the main clause comes first, the comma is often omitted:

• Vou levar um chapéu e outra camisa se fizer sol.

Yes. This is also correct:

• Vou levar um chapéu e outra camisa se fizer sol.

Both versions are natural:

• Se fizer sol, vou levar um chapéu e outra camisa.
• Vou levar um chapéu e outra camisa se fizer sol.

Putting se fizer sol first gives a little more emphasis to the condition.

Because Portuguese often drops subject pronouns when the verb already makes the subject clear.

• vou levar already clearly means I will take
• so eu is not necessary

You could say:

• Se fizer sol, eu vou levar um chapéu e outra camisa.

That is also correct, but the eu is optional here.

It depends on the kind of shirt.

In Brazil:

• camisa often means a shirt more generally, especially a button-up shirt, jersey, or a standard shirt in many contexts
• camiseta often means a T-shirt

So if the speaker means a general shirt or a change of clothes, camisa is fine.
If they specifically mean a T-shirt, camiseta might be more precise.

Not normally. The usual expression is fazer sol, not estar sol.

Natural options include:

• Se fizer sol...
• Se estiver ensolarado... = if it’s sunny

So Se estiver sol is not the standard expression.

A more formal version could be:

• Se fizer sol, levarei um chapéu e outra camisa.

This uses the simple future levarei instead of vou levar.

Both are correct, but in everyday Brazilian Portuguese, the original sentence sounds more natural:

• Se fizer sol, vou levar um chapéu e outra camisa.