Se mia gorĝo denove doloros, mi iros al la apoteko post la laboro.

Esperanto does not follow the English rule that avoids the future tense after if. If the meaning is future, Esperanto can simply use the future ending -os.

So here:

• Se mia gorĝo denove doloros = if my throat hurts / will hurt again
• mi iros = I will go

In other words, Esperanto uses the tense that matches the time being talked about.

Iros is the normal future tense: will go. It fits a real future possibility.

The sentence means: if this happens, then I will go.

Irus is the conditional form: would go. That usually sounds more hypothetical. For example:

• Se mia gorĝo denove dolorus, mi irus al la apoteko.

That is closer to If my throat were to hurt again, I would go to the pharmacy.

So in your sentence, iros is the straightforward choice.

With body parts, Esperanto often uses the body part itself as the subject:

• Mia gorĝo doloras = my throat hurts
• Mia kapo doloras = my head hurts

So dolori here works like to ache / to hurt.

Esperanto can also use dolori in a different pattern, where something hurts a person:

• Tio doloras min = that hurts me

But in this sentence, the natural way is mia gorĝo doloros.

Because gorĝo is the subject, not the direct object.

The accusative ending -n is usually for a direct object. Here:

• mia gorĝo = the thing that will hurt
• mi = the person who will go

So gorĝo stays as gorĝo, not gorĝon.

Denove means again. It is an adverb, so it modifies the verb:

• mia gorĝo denove doloros = my throat will hurt again

Its position is somewhat flexible, but the sentence’s wording is very natural. You may also see things like:

• Se denove mia gorĝo doloros...

That is understandable, but Se mia gorĝo denove doloros... is the more ordinary phrasing.

Al means to and shows movement toward a destination.

So:

• iri al la apoteko = to go to the pharmacy

Because al already expresses direction, you do not add -n to apoteko here. So:

• al la apoteko = correct
• al la apotekon = not correct

La is the definite article, meaning the.

• la apoteko = the pharmacy
• la laboro = the work / the job / the workday, depending on context

In post la laboro, Esperanto uses la even where English often says simply after work. That is normal. The phrase usually means after the workday or after I finish work.

Because post and poste are different kinds of words.

• post = a preposition, meaning after, and it must be followed by something
• poste = an adverb, meaning afterward(s), later

So:

• post la laboro = after work
• Mi laboros, kaj poste mi iros al la apoteko. = I will work, and afterward I will go to the pharmacy.

In your sentence, a noun phrase follows, so post is the right form.

Only if you want a different meaning.

• se = if
• kiam = when

So:

• Se mia gorĝo denove doloros... = If my throat hurts again...
• Kiam mia gorĝo denove doloros... = When my throat hurts again...

Using kiam makes the event sound more certain.

Yes. A comma is normally used to separate the se clause from the main clause.

So:

• Se mia gorĝo denove doloros, mi iros al la apoteko post la laboro.

That is standard written Esperanto and makes the sentence easier to read.

Yes. Esperanto word order is fairly flexible.

For example, you could also say:

• Mi iros al la apoteko post la laboro, se mia gorĝo denove doloros.

That means the same thing. The original version puts the condition first, which is very natural because it sets up the situation before giving the result.