Quando a febre passar, eu vou voltar ao trabalho.

Because after quando referring to a future event, Portuguese normally uses the future subjunctive.

• Quando a febre passar... = When the fever goes away / when the fever has passed
• This is the standard pattern for future time clauses:
  • quando eu chegar = when I arrive
  • quando você puder = when you can
  • quando eles terminarem = when they finish

A native English speaker often expects a future form here, but Portuguese does not usually say quando a febre passará in this kind of sentence.

The future subjunctive is a verb form Portuguese uses mainly after words like:

• quando = when
• se = if
• assim que = as soon as
• logo que = once / as soon as

It is used when the action is not completed yet and is still in the future.

In this sentence:

• passar = future subjunctive of passar

More examples:

• Se eu tiver tempo, eu vou. = If I have time, I’ll go.
• Quando ele chegar, me avise. = When he arrives, let me know.

For many -ar verbs, the future subjunctive form looks the same as the infinitive, so passar can be both:

• infinitive: to pass
• future subjunctive: when it passes

Here, passar means to pass, to go away, or to subside.

So Quando a febre passar means:

• When the fever goes away
• When the fever passes
• When the fever subsides

This is a very common use in Portuguese for temporary conditions:

• A dor passou. = The pain went away.
• A gripe vai passar. = The flu will pass.
• A tristeza passa. = Sadness passes.

Because febre is a feminine noun, and Portuguese often uses the definite article where English might not.

• a febre = the fever

In English, we might say when the fever passes or just think of fever more generally, but in Portuguese, using the article sounds natural.

This is very common:

• a dor = the pain
• a gripe = the flu
• o trabalho = work / the work

So a febre here is just the normal, natural way to say it.

Yes, you absolutely could say:

• Quando a febre passar, vou voltar ao trabalho.

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

• eu vou voltar = I am going to return
• vou voltar = (I) am going to return

Including eu can:

• add emphasis
• make the sentence clearer
• sound slightly more explicit

So both are correct. The version with eu is just a bit more spelled out.

Both are correct, but vou voltar is very common in everyday Brazilian Portuguese.

• vou voltar = I’m going to return / I will return
• voltarei = I will return

In Brazil, the ir + infinitive structure is often more common in spoken language than the simple future.

So:

• Quando a febre passar, eu vou voltar ao trabalho.
  sounds very natural and conversational.

• Quando a febre passar, voltarei ao trabalho.
  is also correct, but can sound more formal, literary, or less conversational depending on context.

Both can mean to return, but voltar is more common and natural in everyday speech.

• voltar ao trabalho = return to work / go back to work
• retornar ao trabalho = return to work

Retornar often sounds a bit more formal or official.

So in normal conversation, Brazilians are more likely to say:

• voltar ao trabalho
• voltar para o trabalho in some contexts, though voltar ao trabalho is especially natural for returning to work after illness, vacation, leave, etc.

Because ao is the contraction of:

• a
  • o = ao

Here, voltar a is followed by o trabalho:

• voltar a o trabalho → voltar ao trabalho

So:

• ao trabalho = to the work / to work

This contraction is mandatory in standard Portuguese.

Similar examples:

• vou ao mercado = I’m going to the market
• cheguei ao escritório = I arrived at the office

Do not confuse this with a trabalho, which means for work or on business:

• Viajei a trabalho. = I traveled for work.

That is a different expression.

It can imply both, depending on context, but here it usually means back to work in the general sense of resuming one’s job.

• voltar ao trabalho often means go back to working / return to one’s job
• It can also suggest physically going back to the workplace, depending on the situation

So if someone has been sick, this sentence usually means:

• I’ll go back to work once the fever is gone
• in other words, I’ll resume working

Because Quando a febre passar is an introductory subordinate clause.

Portuguese usually separates this kind of clause from the main clause with a comma when it comes first:

• Quando a febre passar, eu vou voltar ao trabalho.

If the order is reversed, the comma is often omitted:

• Eu vou voltar ao trabalho quando a febre passar.

Both versions are correct; the first just puts the time condition first.

Not in standard Portuguese for this structure.

• Quando a febre passar, eu vou voltar ao trabalho. = correct
• Quando a febre vai passar, eu vou voltar ao trabalho. = not natural here

After quando referring to a future event, Portuguese normally uses the future subjunctive, not ir + infinitive.

However, Quando a febre vai passar? can appear as a direct question meaning:

• When is the fever going to go away?

So the issue is not that vai passar is always wrong. It is just wrong in this type of future time clause.

Yes. Portuguese allows some flexibility.

These are all natural:

• Quando a febre passar, eu vou voltar ao trabalho.
• Eu vou voltar ao trabalho quando a febre passar.
• Quando a febre passar, vou voltar ao trabalho.

The meaning stays basically the same. The main difference is emphasis:

• starting with Quando a febre passar highlights the condition/time first
• starting with Eu vou voltar ao trabalho highlights the main action first

The original sentence already sounds natural. But in casual speech, a Brazilian might also say:

• Quando a febre passar, vou voltar pro trabalho.
• Quando eu melhorar, vou voltar ao trabalho.
• Assim que a febre passar, vou voltar ao trabalho.

A small note:

• para o often becomes pro in speech:
  • para o trabalho → pro trabalho

But ao trabalho is still perfectly natural and correct, especially in more careful speech or writing.

A rough pronunciation guide is:

KWAN-doo a FEH-vree pah-SAR, eh-oo voh vohl-TAR ow trah-BAH-lyoo

A few notes:

• quando: the qu sounds like kw
• febre: the re at the end is light
• passar: stress on the last syllable, sar
• voltar: stress on tar
• ao sounds like one syllable, roughly ow
• trabalho: the lh sounds like the lli in million for many English speakers

This is only an approximation, but it can help you get started.