O gato aproximou o focinho da porta e começou a miar baixinho.

Because these are two slightly different structures.

• aproximar algo de algo = to bring something close to something
• aproximar-se de algo = to approach something

So:

• O gato aproximou-se da porta = the cat approached the door
• O gato aproximou o focinho da porta = the cat brought its snout/nose close to the door

In your sentence, the focus is specifically on the cat’s snout, not on the whole cat moving toward the door.

Focinho is the snout, muzzle, or nose area of an animal.

For a cat, English might naturally say nose in many contexts, but Portuguese often uses focinho for the animal’s nose/muzzle area. It sounds very normal here.

So aproximou o focinho da porta gives the image of the cat putting its nose right up near the door.

Portuguese often uses the definite article with body parts when the owner is obvious.

So instead of saying his snout, Portuguese commonly says simply the snout:

• O gato aproximou o focinho...

Since it is clearly the cat’s snout, there is no need to say seu focinho.

This is very common in Portuguese:

• levantou a mão = raised his/her hand
• fechou os olhos = closed his/her eyes

English usually prefers a possessive here, but Portuguese often does not.

Da is the contraction of de + a.

• de = of / from / to, depending on the structure
• a = the

So:

• da porta = de a porta

With aproximar algo de algo, Portuguese uses de:

• aproximar o focinho da porta
• literally something like bring the snout near the door

So da porta is required by the verb pattern here.

Because começar is normally followed by a + infinitive.

• começar a fazer = to start doing
• começar a falar = to start speaking
• começar a miar = to start meowing

So:

• começou = started
• a miar = to meow

Together: started to meow

They are in the pretérito perfeito in Portuguese, which often corresponds to the simple past in English.

• aproximou = approached / brought close
• começou = started / began

This tense is commonly used in narration for completed actions in sequence:

1. the cat brought its snout close to the door
2. it started meowing quietly

It moves the story forward.

Baixinho means quietly, softly, or in a very low voice/sound.

It comes from baixo with the diminutive ending -inho, which often adds a sense of:

• smallness
• softness
• gentleness
• slightness

So miar baixinho suggests a soft, gentle, low meow.

You may hear baixo in some contexts, but baixinho is especially natural when talking about sounds made softly.

Here it works as an adverb, modifying miar.

It tells us how the cat meowed:

• miar baixinho = to meow softly/quietly

Even though forms like baixinho can also be adjectives in other contexts, in this sentence it is functioning adverbially.

Portuguese uses definite articles more often than English.

In this sentence:

• O gato = the cat
• o focinho = the snout
• da porta = of the door / near the door

This feels completely natural in Portuguese. English often drops articles or uses possessives where Portuguese keeps the article.

So even if it sounds a bit more article-heavy than English, this is normal Portuguese usage.

Yes, but it means something slightly different.

• O gato aproximou-se da porta... = the cat approached the door
• O gato aproximou o focinho da porta... = the cat brought its snout close to the door

The first version describes the movement of the whole cat. The original sentence gives a more visual, more precise image: the cat put its nose right near the door.

So both are correct, but the original is more specific and vivid.

Yes, it is very natural.

The structure is:

• O gato = subject
• aproximou o focinho da porta = first action
• e começou a miar baixinho = second action

This is a very normal narrative order in Portuguese: subject + action + additional detail + next action.

It sounds like natural written storytelling.

Yes, but the original position is the most straightforward.

• começou a miar baixinho = very natural
• começou a miar muito baixinho = started meowing very softly
• baixinho, começou a miar = possible in a literary style, but much less neutral

Putting baixinho after miar is the normal way to show how the meowing happened.