Quando o sol sair, vamos caminhar no parque.

Because after quando, Portuguese often uses the future subjunctive when talking about a future event.

So in Quando o sol sair, sair is the future subjunctive form of sair.

Compare:

• Quando o sol sai, eu fico feliz. = When/whenever the sun comes out, I get happy.
  
  • Here it sounds more like a general or repeated situation.
• Quando o sol sair, vamos caminhar. = When the sun comes out, we’re going to walk.
  
  • Here it refers to a specific future moment.

A tricky detail: for many verbs, the future subjunctive looks like the infinitive. That is why sair stays sair here.

It means when the sun comes out or sometimes when the sun rises, depending on context.

In everyday Brazilian Portuguese, o sol sair can suggest:

• the sun appearing from behind clouds
• the sun coming out after bad weather
• sometimes the sun rising, if the context supports that

If you specifically want sunrise, Portuguese often uses:

• quando o sol nascer
• ao nascer do sol = at sunrise

So o sol sair is natural, but it is a little broader than just to rise.

In Brazilian Portuguese, ir + infinitive is very common for future meaning, especially in everyday speech.

So:

• vamos caminhar = we are going to walk / we’ll walk
• caminharemos = we will walk

Both are correct, but vamos caminhar sounds more natural and conversational in Brazil.

So this sentence is very typical spoken Brazilian Portuguese.

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

In vamos caminhar, the form vamos already tells you the subject is we.

So both are possible:

• Quando o sol sair, vamos caminhar no parque.
• Quando o sol sair, nós vamos caminhar no parque.

The version without nós is very natural and common.

No is a contraction of:

• em + o = no

So:

• no parque = in the park / at the park

Portuguese uses these contractions all the time:

• em + a = na
• em + os = nos
• em + as = nas

So em o parque is not how standard Portuguese says it. The normal form is no parque.

Because Portuguese uses articles more often than English.

In English, we might simply say in the park, and the article is built into the phrase. In Portuguese, the article is still very active grammatically, so you get:

• em o parque → no parque

You will notice this a lot in Portuguese with places and nouns in general.

Because Quando o sol sair is an introductory subordinate clause.

When that time clause comes first, Portuguese normally separates it with a comma:

• Quando o sol sair, vamos caminhar no parque.

If the order is reversed, the comma is usually not needed:

• Vamos caminhar no parque quando o sol sair.

So the comma here is standard and expected.

Yes. Quando can mean either when or whenever, depending on context.

Examples:

• Quando ele chega, eu saio. = When/Whenever he arrives, I leave.
• Quando ele chegar, eu sairei. = When he arrives, I will leave.

In your sentence, because the verb after quando is in the future subjunctive and the second clause talks about a future plan, the meaning is clearly when in a future sense.

Yes, absolutely.

That version uses the simple future:

• caminharemos

So both are correct:

• Quando o sol sair, vamos caminhar no parque.
• Quando o sol sair, caminharemos no parque.

In Brazilian Portuguese, the first one is usually more common in everyday speech. The second may sound a bit more formal or written.

Both can relate to walking, but they are not exactly the same.

• caminhar usually means to walk in the sense of taking a walk, walking for exercise, or walking some distance
• andar is broader and can mean to walk, to go around, to move, or even to function in some contexts

So in a sentence about going for a walk in the park, caminhar is a very natural choice.

Examples:

• Vamos caminhar no parque. = Let’s go walk in the park.
• Eu ando muito no trabalho. = I walk a lot at work.

No. No parque means in the park or at the park, not to the park.

If you want to the park, you would normally say:

• ao parque

Compare:

• Vamos caminhar no parque. = We’re going to walk in the park.
• Vamos ao parque. = We’re going to the park.

So no parque focuses on the location where the walking happens.

In this sentence, yes, o sol is the normal form.

Portuguese generally uses the definite article with nouns like this, including many natural things and everyday nouns:

• o sol
• a lua
• o céu

So sol sair without o would sound wrong here. The normal phrase is o sol sair.

Yes, it is natural.

It sounds like normal Brazilian Portuguese, especially because of:

• quando + future subjunctive → quando o sol sair
• vamos + infinitive for future plans → vamos caminhar
• omission of nós
• everyday prepositional contraction → no parque

A Brazilian speaker could definitely say this in normal conversation.