Antes de dormir, eu sempre lembro de trancar a porta da varanda.

Because antes de + infinitive is a very common way to say before doing something.

In this sentence, dormir is an infinitive, and the subject is understood to be the same person as in the main clause: eu.

So:

• Antes de dormir = Before going to sleep / Before sleeping

You can say antes de eu dormir, but that is more explicit and is usually used when you want to clearly state the subject or contrast it with someone else.

Compare:

• Antes de dormir, eu tranco a porta. = Before going to sleep, I lock the door.
• Antes de eu dormir, minha mãe verifica as janelas. = Before I go to sleep, my mother checks the windows.

So in your sentence, antes de dormir is the most natural choice.

Because Antes de dormir is an introductory adverbial phrase.

In English, we often also use a comma after an opening phrase like Before going to sleep, ... Portuguese works similarly here.

So:

• Antes de dormir, eu sempre lembro de trancar a porta da varanda.

The comma helps separate the time expression from the main clause.

You may sometimes see short introductory phrases without commas in informal writing, but with a phrase like this, the comma is standard and natural.

Yes. In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear.

So these are both natural:

• Antes de dormir, eu sempre lembro de trancar a porta da varanda.
• Antes de dormir, sempre lembro de trancar a porta da varanda.

Why is it clear? Because lembro is the 1st person singular form, so it already means I remember.

Including eu can add:

• emphasis
• contrast
• clarity in some contexts

But in many everyday sentences, leaving it out is very common.

Because the verb lembrar, when followed by an infinitive or a noun in this kind of meaning, commonly takes the preposition de.

So:

• lembrar de fazer algo = to remember to do something
• lembro de trancar = I remember to lock

This is a matter of verb pattern, sometimes called verb regency.

Examples:

• Lembro de fechar a janela.
• Ela lembrou de ligar para o médico.
• Você lembrou de comprar pão?

In everyday Brazilian Portuguese, lembrar de is extremely common.

In this sentence, it means I remember to lock it.

That is the most natural interpretation of:

• lembro de + infinitive

So:

• eu sempre lembro de trancar a porta = I always remember to lock the door

If you want to say I remember having locked the door, Portuguese usually expresses that differently, for example:

• Lembro de ter trancado a porta. = I remember having locked the door.
• Lembro que tranquei a porta. = I remember that I locked the door.

So the sentence you were given is about not forgetting to do it, not about recalling a past event.

This is an important vocabulary point.

• fechar = to close / shut
• trancar = to lock

So:

• Fechar a porta = close the door
• Trancar a porta = lock the door

A door can be fechada without being trancada.

In your sentence, trancar is used because the speaker is talking about making the door secure, not just shutting it.

It means the balcony door or the door to the balcony/veranda.

Literally:

• porta = door
• da = de + a = of the
• varanda = balcony / veranda

So:

• a porta da varanda = the door of the balcony

In natural English, we would usually say:

• the balcony door
• the door to the balcony

In Brazilian Portuguese, this noun + de + noun structure is very common.

Because da is the contraction of:

• de + a = da

Here, varanda is feminine, and it is being used with the definite article a.

So:

• a varanda = the balcony
• a porta da varanda = the door of the balcony

Portuguese very often uses definite articles in places where English might not.

Other examples:

• o carro do meu pai = my father’s car
• a janela da cozinha = the kitchen window
• a porta da sala = the living room door

Because adverbs like sempre often go before the main verb in Portuguese.

So:

• eu sempre lembro = I always remember

This is the most neutral and natural word order.

You may also hear or read other placements depending on style or emphasis, but sempre before the verb is very common.

For example:

• Eu sempre fecho a janela.
• Ela sempre chega cedo.
• Nós sempre esquecemos isso.

So in your sentence, eu sempre lembro is completely standard.

Yes. Both are possible.

You can hear:

• eu lembro de trancar
• eu me lembro de trancar

Both mean I remember to lock or I remember locking, depending on context.

In Brazilian Portuguese, lembrar and lembrar-se de often overlap in everyday use.

That said:

• lembrar de is very common in speech
• lembrar-se de may sound a little more careful or formal in some contexts

Examples:

• Eu lembro do nome dela.
• Eu me lembro do nome dela.

Both are natural.

Because Portuguese uses definite articles more often than English.

In English, we often say things like:

• lock the door
• brush my teeth
• go to school

Portuguese frequently includes an article where English does too, and sometimes where English would not.

So:

• trancar a porta = lock the door

This sounds complete and natural. Saying just trancar porta would sound incomplete or unnatural in normal standard Portuguese.

Not always exactly, but it is often close enough.

In Brazilian Portuguese, varanda can mean:

• a balcony
• a veranda
• a porch-like outdoor area attached to a building

The exact English translation depends on the kind of house or apartment being described.

So porta da varanda could be:

• balcony door
• veranda door
• door to the patio-like outdoor area

If the meaning was already given to you, just remember that varanda refers to that outdoor attached space.

Yes. There are several natural alternatives, for example:

• Antes de dormir, sempre lembro de trancar a porta da varanda.
• Antes de dormir, eu sempre me lembro de trancar a porta da varanda.
• Eu sempre lembro de trancar a porta da varanda antes de dormir.

These all mean essentially the same thing.

The original sentence sounds very natural and standard. The main differences in these alternatives are just:

• whether eu is stated
• whether me is included
• where the time phrase is placed

So the original is a very good model sentence.