Se o clima estiver bom, vamos caminhar no parque.

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

• Se o clima estiver bom = If the weather is good / if the weather turns out to be good
• estiver is the future subjunctive form of estar

This is very common in Portuguese:

• Se eu tiver tempo, eu vou. = If I have time, I’ll go.
• Se ela puder, ela vem. = If she can, she’ll come.

Using está here would sound wrong for this future-condition meaning.

The future subjunctive is a verb form used mainly when something depends on a future possibility, especially after words like:

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

In this sentence:

• Se o clima estiver bom = If the weather is good

The speaker does not know yet whether the weather will be good, so Portuguese uses the future subjunctive.

More examples:

• Quando ele chegar, sairemos. = When he arrives, we’ll leave.
• Se você quiser, eu ajudo. = If you want, I’ll help.
• Assim que eu terminar, te ligo. = As soon as I finish, I’ll call you.

Estiver comes from the verb estar.

A useful fact: for many verbs, the future subjunctive is based on the third-person plural preterite form.

For estar:

• Preterite, third-person plural: estiveram
• Remove -am → estiver-
• Then add the future subjunctive endings:
  • eu estiver
  • você/ele/ela estiver
  • nós estivermos
  • vocês/eles/elas estiverem

So in your sentence:

• o clima = singular
• therefore: estiver

Both are possible, but vamos caminhar is much more natural and common in everyday Brazilian Portuguese.

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

In Brazil, the structure ir + infinitive is very frequently used for the future:

• vou estudar
• vamos sair
• eles vão chegar cedo

So Se o clima estiver bom, vamos caminhar no parque sounds very normal and conversational.

Because bom agrees with clima, and clima is a masculine noun.

• o clima → masculine singular
• therefore: bom

Agreement in Portuguese matters:

• o clima bom
• a temperatura boa
• os dias bons
• as condições boas

Even though weather is not masculine in English, in Portuguese you must follow the grammatical gender of clima.

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

In everyday Brazilian Portuguese:

• tempo is very common for weather
• clima can also be used, but often sounds a bit broader, like climate or general atmospheric conditions

So many Brazilians would naturally say:

• Se o tempo estiver bom, vamos caminhar no parque.

That said, clima in your sentence is still understandable and acceptable. It may just sound slightly less everyday than tempo for some speakers.

No is a contraction of:

• em
  • o = no

So:

• no parque = in the park / at the park

This kind of contraction is extremely common in Portuguese:

• na escola = em + a
• nos livros = em + os
• nas casas = em + as

Since parque usually takes the article o:

• o parque
• em o parque becomes no parque

Yes, but the meaning can shift a little depending on context.

• caminhar = to walk, often clearly meaning walking for exercise or going on foot
• andar = to walk, but also more generally to move around, and it has many other meanings in Portuguese

In this sentence, caminhar sounds very natural if the idea is taking a walk in the park.

• Se o clima estiver bom, vamos caminhar no parque. = very natural

You could also hear:

• vamos andar no parque

But caminhar often sounds a bit clearer and more specifically like go for a walk.

Yes. Portuguese often omits subject pronouns when the verb already makes the subject clear.

So both are correct:

• Se o clima estiver bom, vamos caminhar no parque.
• Se o clima estiver bom, nós vamos caminhar no parque.

The version without nós is more neutral and very common.

Adding nós can give emphasis or contrast, for example:

• Eles vão ficar em casa, mas nós vamos caminhar no parque.

Yes. In Portuguese, when a conditional clause comes before the main clause, a comma is normally used.

So this is standard:

• Se o clima estiver bom, vamos caminhar no parque.

If the main clause came first, the comma might be omitted:

• Vamos caminhar no parque se o clima estiver bom.

Both are correct; the first just places the condition first.

A natural Brazilian pronunciation would be approximately:

• Se u CLI-ma es-TI-ver bom, VA-mus ca-mi-NHAR nu PAR-ki.

A few helpful pronunciation notes:

• Se o often sounds like seu or siu in connected speech, though it is still written se o
• vamos is often pronounced something like VAMus
• no sounds like nu in many Brazilian accents
• parque usually sounds roughly like PAR-ki

The stress is on:

• cliMA
• esTIver
• ca-miNHAR
• PARque

Not really. Even though the sentence refers to the future, Portuguese does not use a direct equivalent of will be after if here.

English also usually says:

• If the weather is good, we’ll walk in the park.

not:

• If the weather will be good... (normally unnatural in English)

Portuguese works similarly in meaning, but grammatically it uses the future subjunctive:

• Se o clima estiver bom

So it is best to think of it as:

• If the weather is good
• If the weather turns out to be good

rather than trying to match each word to English literally.