Ao tirar a fronha, reparei numa pequena fenda na parede.

Ao + infinitive is a very common Portuguese structure meaning when doing, upon doing, or as ... was doing.

So:

• ao tirar a fronha = when taking off the pillowcase / upon removing the pillowcase

It gives the idea that one action happened at the moment another action was taking place.

A few similar examples:

• Ao abrir a porta, vi o João. = When I opened the door, I saw João.
• Ao chegar a casa, liguei à minha mãe. = When I got home, I called my mother.

In this sentence, it sets the scene for what happened next: first the speaker removed the pillowcase, and at that moment noticed the crack.

Because after ao, Portuguese normally uses the infinitive.

So the pattern is:

• ao + infinitive

Examples:

• ao entrar
• ao sair
• ao ver
• ao tirar

If you use a conjugated verb, the structure changes completely. For example:

• Quando tirei a fronha, reparei... = When I took off the pillowcase, I noticed...

That is also correct, but it is a different construction.

Fronha means pillowcase.

It does not mean:

• sheet = lençol
• blanket = manta / cobertor
• duvet cover = capa de edredão

So in this sentence, the person is taking off the pillowcase, not some other bedding item.

Reparei is the 1st person singular of the pretérito perfeito simples of reparar em.

That means:

• eu reparei = I noticed

It refers to a completed action in the past.

Mini-conjugation:

• eu reparei
• tu reparaste
• ele/ela reparou
• nós reparámos
• vós reparastes
• eles/elas repararam

In European Portuguese, this tense is extremely common for past completed actions.

Because the verb here is reparar em, which means to notice.

So the structure is:

• reparar em algo = to notice something

That is why we get:

• reparei em uma pequena fenda

And in Portuguese, em + uma contracts to numa:

• em + uma = numa

So:

• reparei numa pequena fenda = I noticed a small crack

This is very important because reparar does not always mean the same thing:

• reparar em = to notice
• reparar = to repair/fix in some contexts, though consertar or arranjar are often more common depending on region and context

Yes, very common.

In European Portuguese, reparar em is a very natural everyday way to say to notice.

Other possibilities include:

• notar = to notice
• ver = to see
• dar conta de = to realize / notice

But reparar em often sounds very idiomatic in Portugal.

Examples:

• Não reparei nisso. = I didn’t notice that.
• Reparaste no carro novo dele? = Did you notice his new car?

So the sentence sounds very natural for European Portuguese.

Numa is simply the contraction of:

• em + uma

Since fenda is feminine singular, you use uma:

• uma fenda

And because reparar takes em, you get:

• em uma fenda → numa fenda

Then the adjective agrees too:

• pequena because fenda is feminine singular

So:

• numa pequena fenda = in a small crack / more naturally in English here, a small crack

Fenda means something like crack, split, or narrow opening.

In this sentence, uma pequena fenda na parede suggests a small visible opening or crack in the wall.

Related words:

• fenda = crack, split, slit
• racha = crack, often more colloquial and sometimes stronger-sounding
• fissura = fissure, often more technical or formal
• greta = crack/crevice, often a small narrow crack

In everyday speech, several of these can overlap, but fenda is a good natural choice here.

Because na is the contraction of:

• em + a = na

So:

• na parede = in the wall / on the wall

Portuguese often uses em where English may use in or on, depending on context.

Here, uma fenda na parede is the standard way to say a crack in the wall.

Examples:

• um buraco na parede = a hole in the wall
• uma mancha na parede = a stain on the wall
• um quadro na parede = a picture on the wall

Because Portuguese often drops subject pronouns when the verb ending already makes the subject clear.

Here:

• reparei clearly means I noticed

So eu is unnecessary unless the speaker wants emphasis or contrast.

You could say:

• Eu reparei numa pequena fenda na parede.

But it is more natural in many contexts to omit eu.

This is very normal in Portuguese.

By default, the subject of ao tirar a fronha is understood to be the same as the subject of the main verb.

So in this sentence, the natural reading is:

• When I took off the pillowcase, I noticed a small crack in the wall.

The same person both:

1. took off the pillowcase
2. noticed the crack

This is a normal feature of Portuguese non-finite clauses like ao + infinitive.

Yes, absolutely.

Both are correct:

• Ao tirar a fronha, reparei numa pequena fenda na parede.
• Quando tirei a fronha, reparei numa pequena fenda na parede.

The difference is mostly stylistic:

• ao tirar sounds a bit more compact and elegant
• quando tirei is more explicit and direct

In everyday speech, both are possible.

In Portuguese, adjectives can often go before or after the noun, but the position may affect style, emphasis, or nuance.

Here:

• uma pequena fenda

sounds very natural and often slightly more descriptive or fluid than:

• uma fenda pequena

Both are possible, but pequena fenda is especially common in this kind of narrative description.

Very roughly:

• uma pequena fenda = a small crack
• uma fenda pequena = a crack that is small

The difference is subtle, and in many cases both work.

In European Portuguese, fronha is pronounced approximately like:

• FRON-ya

The key sound is nh, which is like the ny in canyon.

So:

• nh ≈ ny

Other examples:

• vinho ≈ VEEN-yoo
• banho ≈ BAN-yoo
• tenho ≈ TEN-yoo (rough approximation)

Also remember that European Portuguese often reduces unstressed vowels, so the actual pronunciation may sound more compressed than the spelling suggests.

Yes, it sounds natural and idiomatic in European Portuguese.

Why it sounds natural:

• ao + infinitive is common
• reparar em is a very typical European Portuguese way to say notice
• numa and na are normal contractions
• the vocabulary fits the context well

So this is a good example of natural written or spoken Portuguese from Portugal.