Se as natas estiverem no frigorífico, eu faço o molho agora.

Because Portuguese normally uses the future subjunctive after se when the condition is about something not yet confirmed.

Here, the speaker does not know yet whether the cream is in the fridge. So the structure is:

• Se
  • future subjunctive
• main clause with the result

So:

• Se as natas estiverem no frigorífico... = if the cream is in the fridge / if the cream turns out to be in the fridge

Using estão here would not be the usual choice for this kind of future condition.

Because the subject is as natas, which is third person plural.

The verb must agree with the subject:

• eu estiver
• tu estiveres
• ele/ela estiver
• nós estivermos
• eles/elas estiverem

Since as natas is grammatically plural, the correct form is estiverem.

In European Portuguese, natas is the normal word for cream in many everyday contexts, and it is usually treated as a plural noun.

So Portuguese says:

• as natas estão frias
• as natas estiverem no frigorífico

Even though English uses singular cream, Portuguese uses plural natas.

A singular form nata exists, but it usually means something different, such as a layer or skin, not the normal ingredient you would use in cooking.

Because in Portuguese, the present indicative is very often used to talk about the near future, especially when the action is seen as immediate or already decided.

So:

• eu faço o molho agora

can mean:

• I make the sauce now
• I’ll make the sauce now

In this sentence, agora helps give that sense of immediate action, so the present tense sounds natural.

A future form like farei is possible, but it sounds more formal or less conversational.

It is really about a future result based on a present check.

The logic is:

• first, we find out whether the cream is in the fridge
• if yes, the speaker will make the sauce immediately

So the condition is still open, which is why Portuguese uses future subjunctive in the se clause, even though the action in the second clause happens right away.

That combination is very normal in Portuguese.

Yes. In Portuguese, subject pronouns are often omitted because the verb ending already shows who the subject is.

So both are possible:

• Se as natas estiverem no frigorífico, eu faço o molho agora.
• Se as natas estiverem no frigorífico, faço o molho agora.

The version without eu is often more natural unless you want emphasis, contrast, or clarity.

For example, eu might be kept if you want to stress that I will do it, not someone else.

Because no is a contraction of:

• em
  • o = no

And frigorífico is a masculine singular noun in Portuguese, so:

• em o frigorífico becomes no frigorífico

This is exactly the same kind of contraction you see in:

• na cozinha = em + a
• nos armários = em + os
• nas gavetas = em + as

Also, in European Portuguese, frigorífico is the normal everyday word for fridge.

Yes. Frigorífico is the standard everyday word in Portugal for fridge/refrigerator.

A learner may also hear different words in other varieties of Portuguese, especially in Brazil, such as geladeira. But in European Portuguese, frigorífico is the usual choice.

So this sentence sounds clearly natural for Portugal.

Portuguese uses definite articles much more often than English does.

So where English might simply say:

• cream
• sauce

Portuguese often says:

• as natas
• o molho

This can happen when referring to:

• something specific already known in the situation
• an ingredient being discussed
• a noun treated as a concrete item in context

So the articles here sound normal and natural.

Yes. That is also correct and very natural.

You have a few common options for the main clause:

• faço o molho agora — natural, immediate, conversational
• vou fazer o molho agora — also very natural, perhaps even clearer for learners
• farei o molho agora — correct, but more formal or less everyday

So the original sentence is fine, but vou fazer is another common choice in spoken Portuguese.