Ao chegar a casa, fecho a janela.

Ao + infinitive is a very common Portuguese pattern that means something like when doing, upon doing, or on doing.

So:

• ao chegar = when arriving / upon arriving
• ao chegar a casa = when I arrive home / upon arriving home

It introduces the action as a time frame for the main verb. In this sentence, the closing happens at the moment of arrival, or immediately after it.

This structure is especially common in written or slightly more formal Portuguese.

Yes. Ao is the contraction of a + o.

In modern Portuguese, learners should mainly think of ao + infinitive as a fixed, useful pattern:

• ao sair = when leaving
• ao entrar = when entering
• ao chegar = when arriving

So even though it is historically a contraction, the most important thing is to recognise the whole pattern and its meaning.

Because casa here means home, and in that meaning Portuguese normally does not use the article.

So:

• chegar a casa = arrive home

If you were talking about a specific house as a building, then an article could appear:

• chegar à casa da Maria = arrive at Maria’s house

Also, in standard European Portuguese, the verb chegar normally takes the preposition a for destination:

• chegar a casa = arrive home
• chegar à estação = arrive at the station

Compare:

• em casa = at home
• para casa = homewards / to home

So the sentence is correct as chegar a casa.

No.

They look the same, but they do different jobs:

• In chegar a casa, a is a preposition required by chegar
• In a janela, a is the definite article meaning the

So:

• a casa here = to home
• a janela = the window

This is a very common source of confusion for English speakers, because English does not use a/the and to in forms that look identical.

Fecho is the 1st person singular present indicative of fechar.

• eu fecho = I close

In this sentence, the present tense most naturally suggests a habit or routine:

• Ao chegar a casa, fecho a janela.
• When I get home, I close the window.

If you wanted to describe one completed event in the past, you would normally use the past tense:

• Ao chegar a casa, fechei a janela.
• When I got home, I closed the window.

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

Here, fecho already tells you the subject is I.

So both are possible:

• Fecho a janela = I close the window
• Eu fecho a janela = I close the window

The version without eu is more neutral and natural unless you want emphasis or contrast.

For example:

• Eu fecho a janela, mas tu abres a porta.
• I close the window, but you open the door.

Because Portuguese usually uses an article with a specific concrete noun like this.

• a janela = the window
• uma janela = a window

In this sentence, the speaker is talking about a specific window that is understood from the situation, so a janela is the natural choice.

Leaving the article out would sound unnatural in standard Portuguese here.

Yes. That is perfectly natural.

Compare the two:

• Ao chegar a casa, fecho a janela.
• Quando chego a casa, fecho a janela.

Both are correct, but they feel slightly different:

• ao chegar a casa is a bit more compact and slightly more formal or written
• quando chego a casa is very common and straightforward in everyday language

So a learner can safely use either one.

Because Ao chegar a casa is an introductory time phrase placed at the beginning of the sentence.

The comma helps separate that introductory part from the main clause:

• Ao chegar a casa, fecho a janela.

This is very common and recommended in writing. It makes the sentence clearer and easier to read.

Yes. You can say:

• Fecho a janela ao chegar a casa.

That is also grammatical.

The difference is mostly one of focus:

• Ao chegar a casa, fecho a janela.
  This puts the time setting first.

• Fecho a janela ao chegar a casa.
  This puts the main action first.

Both are possible, but the original version sounds especially natural if you want to frame everything around the moment of arriving home.