Se a picada continuar vermelha, vou comprar uma pomada na farmácia.

In this sentence, continuar is the future subjunctive form, which is used very often after se when talking about a possible future situation:

• Se a picada continuar vermelha... = If the bite stays red...

This is a very important pattern in Portuguese:

• Se eu tiver tempo... = If I have time...
• Se ele for... = If he goes...
• Se nós pudermos... = If we can...

For many -ar verbs, the future subjunctive looks exactly the same as the infinitive, so continuar can be confusing because it looks like the dictionary form. But here it is functioning as the correct tense/mood after se.

So:

• se + future possibility → often future subjunctive
• continuar here is correct

Because vermelha agrees with a picada, which is a feminine singular noun.

In Portuguese, adjectives usually agree in gender and number with the noun they describe:

• a picada vermelha = the red bite/sting
• o braço vermelho = the red arm
• as picadas vermelhas = the red bites/stings

Even though in English we do not change red, Portuguese does.

Picada usually means a bite or sting, especially from an insect.

Depending on context, it can refer to:

• a mosquito bite
• an insect sting
• a small puncture-like bite mark

So in this sentence, a picada most naturally means something like:

• the bite
• the sting
• the insect bite

The exact English word depends on the situation.

These mean different things:

• ficar vermelha = to become red
• continuar vermelha = to remain/stay red

So:

• Se a picada ficar vermelha... = If the bite becomes red...
• Se a picada continuar vermelha... = If the bite stays red...

In your sentence, the idea is that the bite is already red, and if that condition continues, the speaker will buy some ointment.

Vou comprar is the near future construction:

• vou + infinitive

It is extremely common in everyday Portuguese and often sounds more natural in conversation than the simple future.

So:

• vou comprar = I’m going to buy
• comprarei = I will buy

Both are grammatically correct, but vou comprar is more common in normal speech.

In European Portuguese, both forms exist, but learners will hear ir + infinitive a lot.

Uma pomada means an ointment or a cream/ointment.

Portuguese usually uses an article where English also would:

• comprar uma pomada = buy an ointment

It tells us the item is indefinite and not previously specified. The speaker is not referring to one particular ointment already known to both people, just some ointment.

Compare:

• uma pomada = an ointment, some ointment
• a pomada = the ointment

Na is a contraction of:

• em + a = na

So:

• na farmácia literally means in/at the pharmacy

In this sentence, it means the place where the speaker will buy the ointment:

• vou comprar uma pomada na farmácia = I’m going to buy an ointment at the pharmacy

This is different from:

• vou à farmácia = I’m going to the pharmacy

So:

• na farmácia focuses on the place of purchase
• à farmácia focuses on movement toward the place

Yes, absolutely.

Both are natural, but the emphasis is slightly different:

• Vou comprar uma pomada na farmácia.
  Focus: buying the ointment, with the pharmacy as the place.

• Vou à farmácia comprar uma pomada.
  Focus: going to the pharmacy in order to buy the ointment.

Both are good Portuguese.

Here, se means if.

It introduces a condition:

• Se a picada continuar vermelha... = If the bite stays red...

This is one of the most common uses of se in Portuguese.

Be careful, though: se can also have other meanings in different contexts, such as a reflexive pronoun:

• Ele lava-se. = He washes himself.

But in your sentence, it simply means if.

Not exactly. Pomada usually means ointment, and sometimes medicated cream, depending on context.

In everyday translation, English speakers might say:

• ointment
• cream
• medicated cream

But pomada often suggests something a bit thicker or more medicinal than a generic cream.

A learner should remember:

• pomada = usually ointment
• creme = usually cream

That said, in real life, the exact translation can depend on the product.

Yes, it sounds natural.

A European Portuguese speaker would understand it immediately, and it is a normal everyday kind of sentence.

You might also hear small variations, such as:

• Se a picada continuar vermelha, vou comprar pomada na farmácia.
• Se a picada não passar, vou comprar uma pomada na farmácia.
• Se continuar vermelha, vou à farmácia comprar uma pomada.

But your original sentence is perfectly natural.

A useful point here is the lh in vermelha. In Portuguese, lh is a single sound, like the lli in some pronunciations of million, though not exactly the same in every accent.

Roughly:

• vermelha ≈ ver-MEH-lya

In European Portuguese, vowels are often reduced more than in Brazilian Portuguese, so the real pronunciation may sound more compressed than an English speaker expects.

The important thing is:

• lh is not pronounced like separate l + h
• it is one palatal sound: lh

The same happens in words like:

• filho
• mulher
• trabalho

Yes. Picada can have several meanings depending on context, such as:

• bite
• sting
• puncture
• injection mark
• chopped/minced mixture in some food contexts

But in a sentence with vermelha and pomada na farmácia, the medical/everyday meaning is clearly the relevant one: a bite or sting.

So context is what tells you which meaning is intended.