Quando faz sol, eu gosto de caminhar no parque.

In this sentence, quando can be understood as when or whenever. Because the sentence talks about a general habit, English often uses whenever naturally, but Portuguese still just uses quando.

So Quando faz sol, eu gosto de caminhar no parque describes something you generally like to do on sunny days.

For a specific future situation, Portuguese would often change the verb form: Quando fizer sol, vou caminhar no parque. That means When it’s sunny / when the sun comes out, I’ll go walk in the park.

Portuguese commonly uses fazer in weather expressions. So faz sol is a standard way to say that it is sunny.

Other common examples:

• Faz calor = It’s hot
• Faz frio = It’s cold
• Faz vento = It’s windy

This is just a normal Portuguese pattern. English uses it is for weather, but Portuguese often uses an impersonal fazer instead.

Because fazer is being used impersonally here. There is no real subject doing the action. Portuguese uses the third-person singular form in these weather expressions:

• Faz sol
• Faz frio
• Faz calor

English uses a dummy subject, it, as in it is sunny, but Portuguese does not need that kind of subject here.

The comma separates the introductory time clause from the main clause.

• Quando faz sol, eu gosto de caminhar no parque.

This is very natural and standard. It works like English: When it’s sunny, I like to walk in the park.

If you put the time clause at the end, the comma is often left out: Eu gosto de caminhar no parque quando faz sol.

Both versions are correct. The version with quando faz sol first gives that idea a little more emphasis.

That’s true: Portuguese often leaves out subject pronouns when the verb already makes the subject clear.

So both of these are natural:

• Eu gosto de caminhar no parque.
• Gosto de caminhar no parque.

Including eu can add clarity, emphasis, or contrast. It can also be helpful in learning materials. In everyday Brazilian Portuguese, dropping eu here is very common.

Because gostar normally requires the preposition de.

So you say:

• gosto de caminhar
• gosto de música
• gosto de café

Not:

• gosto caminhar

This is something you simply have to learn with the verb: gostar de.

After gostar de, Portuguese commonly uses the infinitive to talk about an action in general.

So:

• Eu gosto de caminhar = I like walking / I like to walk

This is similar to English, where both I like walking and I like to walk are possible. Portuguese does not use a form like English walking here; it uses the infinitive caminhar.

They are related, but not identical:

• caminhar = to walk, often in a deliberate or healthy/exercise sense
• andar = to walk, go around, move; more general
• passear = to stroll, go out for leisure, take a walk for pleasure

In this sentence, caminhar sounds very natural if the idea is walking as an activity.
If you said passear no parque, it would sound more like strolling around and enjoying the park.
If you said andar no parque, it would be possible, but a bit more general and less specific.

Because no is the contraction of em + o.

So:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

Since parque is masculine singular and normally takes o, em o parque becomes no parque.

This kind of contraction is very common in Portuguese and is usually required.

Yes, but they are not exactly the same in style.

• Quando faz sol = very common, standard weather expression
• Quando está ensolarado = more descriptive, a bit more formal or explicit
• Quando tem sol = possible in casual speech, but less neutral as a textbook-style weather expression

For a general, natural sentence, Quando faz sol is an excellent choice.

Yes. This is also perfectly natural:

Eu gosto de caminhar no parque quando faz sol.

The meaning stays basically the same. The difference is mostly one of focus:

• Quando faz sol, eu gosto de caminhar no parque.
  Emphasizes the condition/time first.

• Eu gosto de caminhar no parque quando faz sol.
  Sounds a bit more straightforward and neutral.

Both are good Brazilian Portuguese.