Se a frigideira estiver muito quente, eu vou usar manteiga em vez de óleo.

Because se meaning if often triggers the future subjunctive in Portuguese when the condition refers to a possible future situation.

So:

• Se a frigideira está quente... = sounds wrong for this meaning
• Se a frigideira estiver quente... = If the frying pan is hot... / If the frying pan should be hot...

This is a very common pattern in Portuguese:

• Se eu tiver tempo, eu vou. = If I have time, I’ll go.
• Se chover, a gente fica em casa. = If it rains, we’ll stay home.

English does not have a special verb form here, but Portuguese does.

Eu vou usar is the near future / periphrastic future: ir + infinitive.

It literally looks like I am going to use, and in Brazilian Portuguese it is extremely common in everyday speech.

So:

• Eu vou usar manteiga = I’m going to use butter

A more formal or literary alternative would be:

• usarei manteiga = I will use butter

Both are correct, but vou usar sounds much more natural in normal conversation.

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

So both are natural:

• Eu vou usar manteiga...
• Vou usar manteiga...

Including eu can add a little emphasis or clarity, but it is not necessary.

Portuguese uses definite articles more often than English does.

Here, a frigideira means the frying pan. Even if English might sometimes say just if the pan is very hot, Portuguese often prefers the article.

Also:

• frigideira is a feminine noun
• so the article is a

Examples:

• a panela = the pot
• a faca = the knife
• a manteiga = the butter

Here, manteiga and óleo are being used as substances in a general sense, so Portuguese can leave the article out:

• usar manteiga
• usar óleo

That is very natural.

You may also hear articles in other contexts:

• a manteiga que comprei = the butter I bought
• o óleo de cozinha = the cooking oil

So the difference is not that articles are impossible here, but that without the article the sentence sounds more like talking about the ingredient in general.

Em vez de means instead of.

So:

• manteiga em vez de óleo = butter instead of oil

This is the standard expression for choosing one thing in place of another.

You may also hear ao invés de, but traditionally that means something closer to on the contrary / the opposite of. In modern informal speech many people use it like em vez de, but learners are safest using em vez de in sentences like this one.

Literally, muito quente means very hot.

However, depending on context, it may imply something close to too hot. In this sentence, the idea is probably that the pan is hot enough that the speaker chooses butter instead of oil.

If you want to say too hot more explicitly, a very common option is:

• quente demais

So:

• muito quente = very hot
• quente demais = too hot / excessively hot

In Portuguese, adjectives usually come after the noun or after linking verbs like estar.

Here the structure is:

• a frigideira = the frying pan
• estiver muito quente = is / gets very hot

So quente is not directly sitting next to the noun as in English hot pan. Instead, it is part of the verb phrase:

• the pan is very hot

This is completely normal Portuguese word order.

Yes. You can say:

• Se a frigideira estiver muito quente, eu vou usar manteiga em vez de óleo.
• Eu vou usar manteiga em vez de óleo se a frigideira estiver muito quente.

Both are correct.

A small punctuation note:

• If the se clause comes first, a comma is usually used.
• If it comes second, the comma is usually not needed.

Here, se means if.

That is one of the most common meanings of se in Portuguese:

• Se chover... = If it rains...
• Se você quiser... = If you want...

This can confuse learners because se also has other uses in Portuguese, such as reflexive or impersonal constructions. But in this sentence, it is simply the conjunction if.

Frigideira specifically means frying pan / skillet.

Panela usually means pot or a more general cooking pan, especially one with deeper sides.

So if you really mean a frying pan, frigideira is the best word.

Compare:

• frigideira = frying pan / skillet
• panela = pot / cooking pan

In Brazilian Portuguese, óleo is pronounced roughly like OH-lee-oh, but with Portuguese sounds.

A few useful points:

• The stress is on the first syllable: Ó-leo
• There is an accent mark on ó, which shows the stressed vowel
• Many English speakers want to compress it too much, but it has three vowel sounds

Likewise:

• frigideira has stress on dei
• estiver has stress on the last syllable: es-ti-VER

You do not need perfect pronunciation immediately, but noticing the stress helps a lot.

Yes, it sounds natural.

A Brazilian speaker might also say slightly different versions depending on style or nuance, for example:

• Se a frigideira estiver quente demais, vou usar manteiga em vez de óleo.
• Se a frigideira estiver muito quente, vou usar manteiga no lugar do óleo.

But the original sentence is absolutely normal and correct.