A chaleira começou a ferver, e eu tirei o avental para abrir a porta.

Because after começar, Portuguese normally uses a + infinitive to mean to start doing something.

So:

• começou a ferver = started to boil / started boiling

This is a very common pattern:

• começar a falar = to start speaking
• começar a chover = to start raining
• começar a rir = to start laughing

Leaving out the a here would sound wrong.

Because it depends on the verb começou.

In Portuguese, when one verb follows another in this kind of structure, the second verb is often in the infinitive:

• começou a ferver
• começou a correr
• começou a trabalhar

So ferver is not the main conjugated verb of the clause. The conjugated verb is começou.

They describe completed actions in a sequence.

• começou = it started
• tirei = I took off

The sentence presents two events that happened and moved the story forward. That is a classic use of the preterite in Portuguese.

If you used the imperfect instead, the meaning would change. It would sound more like background, habit, or an ongoing situation rather than a completed event.

Portuguese often does drop them, yes. The sentence could also be:

• A chaleira começou a ferver, e tirei o avental para abrir a porta.

But eu is included here for clarity or emphasis.

Since the first clause has a different subject, a chaleira, adding eu makes the switch of subject very clear:

• the kettle started boiling
• I took off the apron

So eu is not required, but it is perfectly natural.

Yes. Tirar has a broad meaning: to remove, to take away, to take off.

With clothing, it is very common:

• tirar o casaco = take off the coat
• tirar os sapatos = take off the shoes
• tirar o avental = take off the apron

So here tirei o avental is a normal way to say I took off the apron.

Because Portuguese does not need a reflexive pronoun here.

If you say:

• tirei o avental

it naturally means I took off the apron I was wearing, unless context suggests otherwise.

English learners sometimes expect something like I removed myself the apron, but Portuguese does not work that way here. In fact, tirei-me o avental would sound unusual in this context.

Portuguese uses definite articles more often than English.

In this sentence, the nouns are treated as specific, identifiable things in the situation:

• a chaleira = the kettle
• o avental = the apron
• a porta = the door

This is very normal in Portuguese. English sometimes uses fewer articles, but Portuguese prefers them in many everyday contexts.

It expresses purpose.

• para + infinitive often means in order to... or simply to...

So:

• tirei o avental para abrir a porta

means that taking off the apron was done for the purpose of opening the door.

It attaches to the action tirei, not to o avental.

Because a porta is the direct object of abrir.

• abrir a porta = to open the door

But à porta means something like at the door or to the door, depending on context:

• Estou à porta. = I am at the door.

So here à porta would be wrong, because the sentence is about opening the door, not being at the door.

Because the sentence joins two full clauses, and there is a change of subject:

• A chaleira começou a ferver
• eu tirei o avental para abrir a porta

In Portuguese, a comma before e is not always used, but it is common and natural when there is a clear pause or when two independent clauses are being linked, especially with different subjects.

So the comma here helps separate the two actions clearly.

Yes, that is perfectly normal.

Just like in English, Portuguese often uses the container to refer to what is inside it. So:

• A chaleira começou a ferver

is a natural everyday way to speak. It does not mean the metal kettle itself is literally boiling; it means the water in it has started to boil.

Yes, avental is masculine:

• o avental

The ending does not always tell you the gender reliably, so it is best to learn nouns together with their article:

• o avental
• a chaleira
• a porta

That helps you remember both the word and its gender at the same time.