Quando faz calor, eu prefiro iogurte com morangos.

Because fazer is commonly used in Portuguese for weather expressions.

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

This is just the normal idiomatic way to talk about weather in many contexts.

Using é calor would sound unnatural here.

You can also hear está quente, but there is a nuance:

• faz calor = the weather is hot
• está quente = it is warm/hot right now, possibly referring to the weather, a room, food, an object, etc.

So in this sentence, Quando faz calor is a very natural way to mean When it’s hot.

In weather expressions, fazer is impersonal, which means it does not have a real subject. Because of that, it stays in the 3rd person singular:

• faz calor
• faz frio
• faz vento

Even though English uses it, Portuguese does not treat this as a normal subject in the same way. So faz is the correct form.

Not always. Portuguese often drops subject pronouns when the verb form already makes the subject clear.

So both are correct:

• Quando faz calor, prefiro iogurte com morangos.
• Quando faz calor, eu prefiro iogurte com morangos.

Adding eu can:

• give a little emphasis
• make the sentence clearer
• sound slightly more explicit or contrastive

For example:

• Ele gosta de sorvete, mas eu prefiro iogurte.

So in your sentence, eu is correct, but not required.

Here, quando means when.

In this sentence, it introduces a general situation:

• Quando faz calor, eu prefiro iogurte com morangos.
• When it’s hot, I prefer yogurt with strawberries.

This is not about one specific moment. It means whenever it is hot or in hot weather.

The sentence starts with a time clause:

• Quando faz calor = When it’s hot

In Portuguese, when this kind of clause comes first, a comma is very common and natural:

• Quando faz calor, eu prefiro iogurte com morangos.

You can also change the order:

• Eu prefiro iogurte com morangos quando faz calor.

That version usually does not need the comma.

So the comma is there because the sentence begins with the quando clause.

Because preferir means to prefer, while gostar de means to like.

Compare:

• Eu gosto de iogurte com morangos. = I like yogurt with strawberries.
• Eu prefiro iogurte com morangos. = I prefer yogurt with strawberries.

So prefiro expresses a choice or preference, not just general enjoyment.

Also note that preferir usually does not need a preposition before the thing you prefer:

• Prefiro iogurte.
• Prefiro café.

If you compare two things, you can say:

• Prefiro iogurte a sorvete. = I prefer yogurt to ice cream.

Both are possible, but they mean slightly different things.

• iogurte com morangos = yogurt with strawberries
  This suggests actual strawberries are included or added.
• iogurte de morango = strawberry yogurt
  This usually refers to the flavor/type.

So:

• com morangos focuses on the strawberries as an addition or topping
• de morango focuses on the yogurt flavor

In your sentence, iogurte com morangos sounds like yogurt served with strawberries.

Because com morangos means with strawberries in the general sense of pieces or multiple strawberries.

That is the natural choice when talking about fruit added to yogurt.

Compare:

• iogurte com morangos = yogurt with strawberries
• iogurte com banana = yogurt with banana
• iogurte com frutas = yogurt with fruit

You might also see singular in some contexts, but here the plural sounds very natural because it suggests actual strawberries as ingredients or topping.

Yes, you can, and it would be understandable:

• Quando está quente, eu prefiro iogurte com morangos.

But Quando faz calor is often more idiomatic when talking about hot weather in general.

A rough distinction:

• Quando faz calor = when the weather is hot
• Quando está quente = when it is hot right now

Both can work, but faz calor is especially common in weather statements.

In Brazilian Portuguese, iogurte is commonly pronounced roughly like:

• yo-GOOR-chee or yo-GOOR-tee, depending on accent

A few helpful notes:

• The stress is on gur: io-GUR-te
• The final te in many Brazilian accents sounds like chee or tee
• The initial io sounds like yo

So a practical learner approximation is:

• yo-GOOR-chee

Yes. Portuguese allows some flexibility here.

Original:

• Quando faz calor, eu prefiro iogurte com morangos.

Another natural version:

• Eu prefiro iogurte com morangos quando faz calor.

Both are correct. The difference is mostly about emphasis:

• starting with Quando faz calor highlights the condition first
• starting with Eu prefiro highlights the speaker’s preference first

So the original sentence is perfectly natural, but it is not the only possible order.

Yes. Calor is a noun meaning heat.

But in the expression fazer calor, Portuguese uses the noun in a fixed weather expression that means to be hot.

So literally, it is something like:

• it makes heat

But you should learn faz calor as a whole expression meaning:

• it’s hot
• the weather is hot

That will sound much more natural than translating word by word.