Na infância, eu tinha medo de falar em público.

Because na is the contraction of em + a.

• infância is a feminine noun
• so em a infância becomes na infância

This is very common in Portuguese:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

So na infância literally has the structure in the childhood, but in natural English it corresponds to in childhood or during childhood.

It means during childhood or in childhood.

It refers to a period of life, not a specific moment. In this sentence, it sets the time frame for the rest of the idea.

Similar Portuguese expressions include:

• na adolescência = in adolescence
• na juventude = in youth
• na velhice = in old age

So Na infância, eu tinha medo... means something like When I was a child / During my childhood, I was afraid...

Tinha is the imperfect form of ter.

Here, the imperfect is used because the sentence describes:

• a past state
• something ongoing or habitual
• not a single completed event

So eu tinha medo means I used to be afraid or I was afraid over a period of time.

That is why tinha works better than a completed past form like tive, which would sound more like a specific, finished occurrence.

Because ter medo here describes an ongoing emotional state in childhood, not one isolated event.

Compare:

• Na infância, eu tinha medo de falar em público.
  = During childhood, I was afraid of speaking in public / I used to be afraid...

• Tive medo de falar em público naquele dia.
  = I was afraid to speak in public that day.

So:

• tinha = ongoing background state
• tive = specific completed instance

This is one of the classic uses of the imperfect in Portuguese.

No, it is not strictly necessary.

Portuguese often drops subject pronouns because the verb form already shows the person:

• (Eu) tinha medo de falar em público.

Both are correct.

Including eu may be used for:

• emphasis
• contrast
• clarity

For example:

• Na infância, eu tinha medo de falar em público, mas o meu irmão não.

In a simple standalone sentence, many native speakers would naturally omit it.

Because ter medo de is the normal Portuguese structure for to be afraid of.

It works like this:

• ter medo de + noun
• ter medo de + infinitive

Examples:

• Tenho medo de cães. = I’m afraid of dogs.
• Tenho medo de falhar. = I’m afraid of failing.

So in your sentence:

• tinha medo de falar em público

the de is required after medo.

Because after medo de, if the next idea is a verb, Portuguese normally uses the infinitive.

So:

• medo de falar
• medo de errar
• medo de cair

This is similar to English afraid of speaking, though Portuguese usually uses the infinitive here rather than a gerund-like form.

So de falar em público means of speaking in public.

No, they are different.

em público means:

• in public
• publicly
• in front of other people

So:

• falar em público = to speak in public / do public speaking

But ao público usually means:

• to the public
• to an audience
• directed toward the public

Compare:

• Tenho medo de falar em público.
  = I’m afraid of speaking in public.

• O político falou ao público.
  = The politician spoke to the public / audience.

So em público is the correct expression here.

You could, but it would mean something slightly different.

• falar em público = speak in public, publicly
• falar para o público = speak to the audience / to the public

In your sentence, the idea is fear of the act of public speaking in general, so falar em público is the most natural choice.

If you say falar para o público, the focus shifts more toward the listeners.

Yes. Portuguese is quite flexible here.

All of these are possible:

• Na infância, eu tinha medo de falar em público.
• Eu tinha medo de falar em público na infância.
• Tinha medo de falar em público na infância.

The version with Na infância at the beginning is very natural because it sets the scene first.

It is especially common when talking about life stages or background information.

Yes, absolutely.

• na infância = in childhood
• durante a infância = during childhood

Both are correct. The difference is mainly stylistic:

• na infância sounds very natural and compact
• durante a infância can sound a little more explicit or slightly more formal

So your original sentence is perfectly natural.

Not exactly.

ter medo usually refers to a more general or lasting fear:

• Tenho medo de alturas.
• Na infância, eu tinha medo de falar em público.

estar com medo often refers more to a temporary feeling at a particular moment:

• Estou com medo agora.
• Ela estava com medo antes da apresentação.

So in this sentence, tinha medo is better because it describes a recurring fear during childhood, not just one moment of fear.

Yes, it is completely natural in European Portuguese.

A native speaker in Portugal would understand it immediately, and it sounds normal as written.

You might also hear close alternatives such as:

• Em criança, tinha medo de falar em público.
• Quando era criança, tinha medo de falar em público.

But your original sentence is correct and idiomatic.

The stress is on the pu syllable:

• PÚ-bli-co

The written accent in público shows that the stress falls there.

That is important because Portuguese stress is not always predictable, and the accent mark tells you where the emphasis goes.

Also, público and publico are not the same form:

• público = public, audience, public space
• publico = I publish

So the accent matters both for pronunciation and meaning.