Learn Hardcore Portuguese (Portugal): Quando era criança, eu sonhava em ser herói e salvar o mundo. - When I was a child, I dreamed of being a hero and saving the world.

Portuguese uses the imperfect tense (era) for situations or states that lasted for a while in the past, especially background information and childhood.

Quando era criança = When I was a child (describes a general period of life, not a single event).
Quando fui criança sounds strange, because being a child is not a single completed action; it’s a continuous state, so the preterite (fui) is not natural here.

So for age, habits, and repeated situations in the past, use the imperfect: era, tinha, morava, brincava, etc.

Could I say Quando eu era criança instead of Quando era criança?

Yes, both are correct:

Quando era criança, eu sonhava…
Quando eu era criança, eu sonhava…

In Portuguese (especially European), the subject pronoun eu is often optional, because the verb ending already shows the person:

era → clearly 1st person singular in this context (I/he/she/it was, but context clarifies)
sonhava → also 1st person singular here

Adding eu can sound a bit more emphatic or personal, but it’s not required.

Why is era used instead of estava in Quando era criança?

Portuguese uses:

ser (era) for permanent or defining characteristics, identities, and long-term states.
estar (estava) for temporary states or conditions.

Being a child is considered a stage of life / identity, not a temporary condition like being tired. So:

Quando era criança (identity / life stage) ✅
Quando estava cansado (temporary condition) ✅

Quando estava criança is incorrect.

Why is sonhava in the imperfect, and not something like sonhei?

Sonhava (imperfect) shows that this was a repeated or habitual action in the past:

eu sonhava em ser herói = I used to dream / I would dream of being a hero.

If you used the preterite:

eu sonhei em ser herói = I dreamed (once / at a specific moment) of being a hero.

Here we are describing a general, recurring childhood habit, so sonhava is the natural choice.

Why is it sonhava em ser and not sonhava ser or sonhava de ser?

In European Portuguese, when you mean “to dream of doing something (as a wish/goal)”, the common structure is:

sonhar em + infinitive
- sonhar em ser herói = to dream of being a hero
- sonhar em viajar pelo mundo = to dream of travelling around the world

Sonhar ser (without em) is not standard in this sense, and sonhar de ser is incorrect.

So eu sonhava em ser herói is the idiomatic way to express an aspiration.

What is the difference between sonhar em and sonhar com?

Both exist, but they are used slightly differently:

sonhar com + noun / person / situation
- often for literal dreams while sleeping, but also for wishes:
- Sonhei com o meu avô. = I dreamed about my grandfather.
- Sonho com uma vida melhor. = I dream of a better life.
sonhar em + infinitive
- very typical for aspirations / plans to do something:
- Sonho em ser médico. = I dream of being a doctor.
- Sonhava em ser herói. = I used to dream of being a hero.

In your sentence, we’re talking about an aspiration, so sonhava em ser fits perfectly.

Why is it ser herói and not ser um herói?

Both are possible, but there is a nuance:

ser herói (without article) treats herói more like a role / identity / profession, similar to:
- ser médico, ser professor, ser escritor.
ser um herói can slightly emphasize a particular hero / one hero, and may sound a bit more dramatic or specific.

In everyday speech, ser herói is very natural and generic: to be a hero (in general).

Could I say Quando era criança, eu sonhava em ser herói e em salvar o mundo?

Yes, that is grammatically correct:

…sonhava em ser herói e em salvar o mundo.

However, in practice Portuguese speakers often omit the second em and only keep it before the first verb:

sonhava em ser herói e salvar o mundo.

The preposition em is understood to apply to both ser and salvar, so repeating it is not necessary.

Why are ser and salvar in the infinitive, not something like fosse or salvasse?

After sonhar em, the default pattern is em + infinitive:

sonhar em ser, sonhar em viajar, sonhar em ganhar, etc.

The forms fosse and salvasse are subjunctive past forms and would be used in other kinds of structures, for example with que:

Quando era criança, eu sonhava que era herói e que salvava o mundo.

Both versions are correct, but:

sonhar em + infinitive is more compact and idiomatic for aspirations.
sonhar que + clause is also possible but slightly different in style.

Why is there an article in salvar o mundo, and not just salvar mundo?

In Portuguese, countable singular nouns almost always need an article unless something special is going on. Mundo (world) is treated as a normal noun:

o mundo = the world

So:

salvar o mundo = to save the world ✅
salvar mundo ❌ (sounds wrong in standard Portuguese)

This also happens in many other expressions: mudar o mundo, viajar pelo mundo, conhecer o mundo, etc.

Is criança masculine or feminine, and does it change if the child is a boy?

Grammatically, criança is feminine:

a criança, uma criança, quando era criança.

It is used for both boys and girls. If you need to specify the sex, you add an adjective or noun:

uma criança do sexo masculino (a male child)
uma criança muito inteligente (gender understood from context)

But the word criança itself always behaves as feminine in grammar.

Is herói masculine? What would be the feminine form?

Yes, herói is the masculine form:

um herói, o herói.

The usual feminine form is:

uma heroína, a heroína.

So a girl could say:

Quando era criança, eu sonhava em ser heroína e salvar o mundo.

Is this sentence natural in European Portuguese, or does it sound Brazilian?

The sentence:

Quando era criança, eu sonhava em ser herói e salvar o mundo.

is perfectly natural in European Portuguese and would also be fully understood and accepted in Brazilian Portuguese.

A European Portuguese speaker might also commonly drop eu:

Quando era criança, sonhava em ser herói e salvar o mundo.

But as written, it’s fine and idiomatic in both varieties.