Se faltar manteiga, eu vou usar óleo.

Because after se meaning if, Portuguese often uses the future subjunctive when the condition refers to the future.

So in Se faltar manteiga..., faltar is the future subjunctive of faltar.

This is confusing for English speakers because for many regular verbs, the future subjunctive looks exactly like the infinitive:

• faltar = infinitive
• faltar = future subjunctive

So the sentence means If butter runs out / if there is no butter..., not simply a present-tense statement.

Here, faltar means something like:

• to be missing
• to be lacking
• to run out
• to not be enough

So Se faltar manteiga can be understood as:

• If there isn’t any butter
• If butter runs out
• If we’re short on butter

Literally, Portuguese structures this more like If butter is lacking than If we lack butter.

In Portuguese, mass nouns like butter, oil, water, rice, etc. often appear without an article when speaking generally.

So:

• faltar manteiga
• usar óleo

sound natural when you mean the ingredient in a general sense.

You could also hear:

• Se faltar a manteiga, eu vou usar o óleo.

That version sounds more like you are talking about the specific butter and the specific oil already known in the situation.

Both are possible, but the version without articles is very natural.

Yes. In Portuguese, subject pronouns are often optional because the verb form usually shows who the subject is.

So both are natural:

• Se faltar manteiga, vou usar óleo.
• Se faltar manteiga, eu vou usar óleo.

Including eu can add:

• emphasis
• clarity
• contrast

For example, eu might be used if the speaker wants to stress what I will do.

Vou usar is the very common ir + infinitive future, especially in spoken Brazilian Portuguese.

So:

• eu vou usar óleo = I’m going to use oil / I will use oil
• eu usarei óleo = I will use oil

Both are correct, but:

• vou usar sounds more natural in everyday speech
• usarei sounds more formal, written, or stiff in many everyday contexts

Brazilian Portuguese strongly prefers vou + infinitive in normal conversation.

Yes. The exact nuance depends on context.

It can mean:

• there is no butter available
• there is not enough butter
• the butter runs out during cooking

Portuguese often leaves that slightly open, and the listener understands it from the situation.

So faltar is broader than a single English translation.

Yes. In this sentence, se means if and introduces a condition:

• Se faltar manteiga, eu vou usar óleo.
• If butter runs out, I’ll use oil.

Just be aware that se can have other uses in Portuguese in other contexts, especially as a reflexive or impersonal particle, but here it is simply the conditional if.

Because the sentence begins with a conditional clause:

• Se faltar manteiga = the condition
• eu vou usar óleo = the main clause

In Portuguese, it is normal to put a comma when this kind of dependent clause comes first.

If you reverse the order, the comma is often omitted:

• Eu vou usar óleo se faltar manteiga.

Both versions are correct.

Yes. That is completely correct and means the same thing.

Compare:

• Se faltar manteiga, eu vou usar óleo.
• Eu vou usar óleo se faltar manteiga.

The difference is mostly about focus and rhythm:

• starting with Se faltar manteiga highlights the condition first
• starting with Eu vou usar óleo highlights the action first

Both are natural.

In Brazilian Portuguese:

• óleo usually means cooking oil in general, often vegetable oil
• azeite usually means olive oil

So vou usar óleo suggests a generic oil substitute, not specifically olive oil.

If the speaker meant olive oil, they would more likely say:

• eu vou usar azeite

This is a vocabulary point, but it matters because English oil can be more ambiguous.

Yes, that is grammatically correct, but it is less natural in everyday speech than Se faltar manteiga.

Compare:

• Se faltar manteiga = more common, natural, conversational
• Se a manteiga faltar = possible, but a bit more marked or formal

Portuguese often prefers the version with faltar + noun in this kind of practical everyday statement.

Óleo is pronounced with the stress on the first syllable: Ó-le-o.

A rough English approximation is OH-lee-oh, but the middle sounds are shorter and more connected in real Portuguese.

Important points:

• the ó is stressed
• it has three syllables
• the accent mark tells you where the stress goes: ó-le-o

So it is Óleo, not o-LE-o.

Yes, very natural.

It sounds like a normal everyday sentence someone might say while cooking or planning what to do if an ingredient is unavailable.

That is one reason it is useful for learners: it shows several very common features of Brazilian Portuguese at once:

• se
  • future condition
• future subjunctive: faltar
• everyday future: vou usar
• optional subject pronoun: eu
• natural use of bare mass nouns: manteiga, óleo