Se faltar alho, eu vou comprar mais no supermercado.

Because after se in a real future condition, Portuguese normally uses the future subjunctive.

So:

• Se faltar alho, eu vou comprar mais. = If we run out of garlic / If garlic is lacking, I’ll buy more.

This is different from:

• Se falta alho, eu compro mais. = more like If garlic is lacking / Whenever we’re short of garlic, I buy more.

So faltar here is used because the sentence is talking about a possible future situation.

It looks like the infinitive, but in this sentence it is actually the future subjunctive form.

For many regular verbs, the future subjunctive has forms that look identical to the infinitive in some persons:

• faltar = infinitive
• se eu faltar
• se ele/ela/você faltar

So in Se faltar alho, the form is future subjunctive, not an infinitive.

Here faltar means something like:

• to be lacking
• to be missing
• to run short
• to run out

With food or supplies, Portuguese often uses faltar where English uses expressions like be out of or not have enough.

So:

• Se faltar alho literally means If garlic is lacking
• Natural English: If we run out of garlic or If there isn’t enough garlic

The grammatical subject is alho.

That can feel a little unusual to an English speaker, because English often says:

• If we run out of garlic

But Portuguese structures it more like:

• If garlic is lacking

So alho is the thing that is missing.

You can see the same pattern in sentences like:

• Falta água. = There’s no water / Water is lacking.
• Faltou energia. = The power went out / Electricity was lacking.

Because alho is being used as an ingredient or substance in a general, non-specific way.

In Portuguese, with food and other uncountable or mass nouns, it is very common to omit the article when talking about quantity or availability:

• Tem leite?
• Preciso comprar pão.
• Faltou arroz.

So faltar alho sounds natural.

If you say o alho, it can sound more specific, like a particular garlic item or the garlic needed for a specific context:

• Se faltar o alho da receita... = If the garlic for the recipe is missing...

But in your sentence, the more natural general wording is alho without the article.

Because alho is understood from the previous clause, so Portuguese can leave it out.

• Se faltar alho, eu vou comprar mais.
• Se faltar alho, eu vou comprar mais alho.

Both are correct.

The first is more natural because it avoids repeating alho unnecessarily. English does the same thing:

• If we run out of garlic, I’ll buy more.

Both are correct, but vou comprar is much more common in everyday Brazilian Portuguese.

• eu vou comprar = near future / very natural spoken Brazilian Portuguese
• comprarei = simple future, correct but often more formal or less common in speech

So:

• Se faltar alho, eu vou comprar mais. sounds very natural in Brazil.
• Se faltar alho, comprarei mais. is grammatical, but less conversational.

Yes. In most cases, you can say:

• Se faltar alho, vou comprar mais no supermercado.

That sounds completely natural.

Portuguese often omits subject pronouns when the verb already makes the subject clear. Since vou clearly means I am going to, eu is optional here.

Including eu may add a little emphasis or clarity, but it is not required.

No is a contraction of:

• em + o = no

So no supermercado literally means:

• in the supermarket
• at the supermarket

In this sentence, the idea is buy more at the supermarket.

It does not literally mean to the supermarket. If you wanted to express movement toward the supermarket, you would usually use:

• ao supermercado = to the supermarket

For example:

• Vou ao supermercado comprar mais alho. = I’m going to the supermarket to buy more garlic.

But your sentence focuses on where the buying happens, so no supermercado is correct.

Because the sentence begins with a subordinate conditional clause:

• Se faltar alho, ... = If garlic runs out, ...

In Portuguese, it is very common to put a comma after an introductory se clause when it comes first.

So this is standard punctuation:

• Se faltar alho, eu vou comprar mais no supermercado.

If the order is reversed, the comma is usually not needed:

• Eu vou comprar mais no supermercado se faltar alho.

Yes, but it changes the feel slightly.

• Se faltar alho = general, natural, about garlic as an ingredient/supply
• Se faltar o alho = more specific, as if you mean the garlic in a particular context

For example:

• Se faltar o alho da receita, improvise.
• Se faltar o alho que eu comprei ontem, avise.

So for a general everyday sentence about running out of garlic, Se faltar alho is the more natural choice.