Ao sair de casa, vi uma poça junto à porta.

Literally, ao sair means something like on leaving or when leaving.

It is formed with:

• a + o = ao
• the infinitive sair = to leave / to go out

So the pattern is:

• ao + infinitive

This is a very common structure in Portuguese for saying when doing something, upon doing something, or while doing something.

Examples:

• Ao entrar, vi-o logo. = When I went in, I saw him immediately.
• Ao chegar a casa, ligou-me. = When he got home, he called me.

In your sentence, Ao sair de casa means When leaving the house / As I was leaving home.

In Portuguese, casa often appears without an article when it means home in a general, personal sense.

So:

• sair de casa = leave home
• estar em casa = be at home
• voltar para casa = go back home

But if you are talking about a specific house as a building, you often use the article:

• sair da casa amarela = leave the yellow house
• a porta da casa = the door of the house

So in Ao sair de casa, casa means home, which is why there is no article.

Because after ao, Portuguese normally uses the infinitive, not a finite verb form.

So you say:

• ao sair = when leaving
• not ao saí

This is just how the structure works grammatically. It functions a bit like an English clause such as when leaving rather than when I left.

Even though the subject is not stated in ao sair, it is understood from the main verb. Here, the person who left home is the same person who saw the puddle.

So:

• Ao sair de casa, vi... means
• When I left home, I saw...

Vi is the 1st person singular preterite of the verb ver (to see).

So:

• ver = to see
• vi = I saw

This is a very common irregular verb.

A few forms of ver in the preterite are:

• eu vi = I saw
• tu viste = you saw
• ele/ela viu = he/she saw
• nós vimos = we saw
• vocês viram = you saw
• eles/elas viram = they saw

In this sentence, vi tells you that the action is completed in the past: I saw a puddle.

Because Portuguese often leaves out subject pronouns when the verb form already makes the subject clear.

Here, vi can only mean I saw, so eu is unnecessary.

That is very normal in Portuguese:

• Vi uma poça. = I saw a puddle.
• Fui ao mercado. = I went to the market.
• Cheguei cedo. = I arrived early.

You can include eu if you want emphasis or contrast, but normally it is omitted.

Poça means puddle.

It is a feminine noun:

• uma poça = a puddle
• a poça = the puddle

It usually refers to a small pool of liquid, especially rainwater, on the ground.

Be careful not to confuse it with similar-looking words:

• poço = well
• poça = puddle

So vi uma poça is simply I saw a puddle.

The ç is called c-cedilha. It shows that the c is pronounced like s before a, o, or u.

So:

• poça is pronounced roughly like POS-sa in European Portuguese, with a short open vowel and an s sound in the middle.
• Without the cedilla, poca would not have the correct pronunciation.

The cedilla is used only before a, o, and u:

• ça, ço, çu

Before e and i, a plain c already has the s sound:

• cedo
• cidade

Junto à porta means next to the door or by the door.

The à is a contraction of:

• a (the preposition used after junto)
• a (the feminine singular definite article the)

So:

• junto a + a porta = junto à porta

This is similar to:

• vou à escola = I’m going to the school
• está à janela = he/she is at the window

Because porta is feminine singular, the contraction is à.

Yes. All of these are possible, but they are not always exactly identical in tone or usage.

• junto à porta = right by / next to the door
• junto da porta = also very natural, often with a very similar meaning
• perto da porta = near the door

In many everyday situations, junto à porta and junto da porta are very close in meaning. Junto often suggests a position very close or directly beside something, while perto de can be a little broader.

So in this sentence, junto à porta gives a nice sense of the puddle being right by the door.

Normally, yes.

In a sentence like this, the subject of the infinitive phrase ao sair de casa is usually understood to be the same as the subject of the main verb.

So:

• Ao sair de casa, vi uma poça...

naturally means:

• When I left home, I saw a puddle...

It would sound odd if one person left home but a different person saw the puddle, unless the sentence were rewritten more explicitly.

Yes, Vi uma poça ao sair de casa is also grammatical.

The version you have:

• Ao sair de casa, vi uma poça junto à porta.

puts the time/context first. This is very common and natural. It sets the scene before giving the main action.

The alternative:

• Vi uma poça ao sair de casa.

is also fine, but it gives the main action first and the time phrase after it.

Both are correct. The original version sounds slightly more narrative and descriptive.

The sentence works in both European Portuguese and Brazilian Portuguese.

However, a learner of Portuguese from Portugal may notice a few things:

• ao + infinitive is very common and natural in European Portuguese
• junto à porta sounds perfectly natural in Portugal
• pronunciation will be quite different in European Portuguese, especially unstressed vowels like the de in de casa and the final vowels in words such as porta

So grammatically the sentence is not exclusive to Portugal, but its pronunciation and some stylistic preferences will reflect European Portuguese usage.