Antes de saírem, beijam-se à porta de casa.

Because this is an example of the personal infinitive, which Portuguese uses when the infinitive has a clear subject.

• antes de sair = before leaving / before one leaves
• antes de saírem = before they leave

Here, the people who leave are clearly they, so Portuguese can mark that in the infinitive itself:

• sair = uninflected infinitive
• saírem = personal infinitive, 3rd person plural

This is very common in Portuguese, and it often feels unusual to English speakers because English infinitives do not change like this.

The personal infinitive is an infinitive that changes form to show who is doing the action.

With sair, the forms are:

• sair = I / you / he / she / we / they leave, in the abstract infinitive sense
• saíres = for you to leave
• sairmos = for us to leave
• saírem = for them / you plural to leave

In this sentence:

• Antes de saírem = Before they leave

Portuguese often uses the personal infinitive after prepositions such as antes de, depois de, por, sem, etc., especially when the subject is explicit or needs to be understood clearly.

In European Portuguese, object pronouns are very often attached to the end of the verb. This is called enclisis.

So:

• beijam-se = they kiss each other / they kiss themselves

In this sentence, the meaning is reciprocal:

• beijam-se = they kiss each other

English speakers often expect something like se beijam, but in standard European Portuguese, beijam-se is the normal form here.

Grammatically, se can do both jobs, depending on context.

With a plural subject like they, it often has a reciprocal meaning:

• Eles beijam-se. = They kiss each other.

It can sometimes also be reflexive:

• Eles lavam-se. = They wash themselves.

In your sentence, beijam-se is naturally understood as they kiss each other, not they kiss themselves.

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

• beijam-se already shows 3rd person plural
• saírem also points to a plural subject

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

Compare:

• Antes de saírem, beijam-se à porta de casa. = neutral, natural
• Antes de saírem, eles beijam-se à porta de casa. = more emphasis on they

This is very normal in Portuguese and much more common than in English.

Portuguese, like English, often uses the present tense for general actions, habitual actions, or vivid description.

So this sentence can mean something like:

• Before leaving, they kiss at the front door
• Before they leave, they kiss at the door of the house

It may describe:

• a habitual action
• a scene in a story
• something happening now or regularly

The present tense here is completely natural.

À porta de casa means at the door of the house/home, often more naturally in English:

• at the front door
• at the doorway
• at the door of the house

It is made up of:

• a = at / to
• a porta = the door
• de casa = of the house / of home

And a + a = à, so:

• a a porta → à porta

That is why there is a grave accent on à.

Because à is the contraction of:

• a = preposition
• a = feminine singular article the

So:

• a + a porta = à porta

This accent is not just a pronunciation mark here; it shows a contraction.

Compare:

• Vou à escola. = I go to the school
• Está à porta. = He/she is at the door

Both are possible in Portuguese, but they do not always sound the same.

• à porta de casa is a very common expression meaning at the door of the house/home, often with a more general or idiomatic sense
• à porta da casa sounds more specifically like at the door of the house, referring to a particular house as an object

In many everyday contexts, de casa is the more natural phrase.

For example:

• Estou a caminho de casa. = I’m on my way home.
• à porta de casa often has that same familiar, idiomatic feel

So in your sentence, à porta de casa is the most natural wording.

They are very close in meaning, but not identical in style.

• Antes de saírem = more compact and elegant
• Antes de eles saírem = more explicit, because it states the subject eles

In many cases, Portuguese prefers the shorter version when the subject is already clear from context.

However, antes de eles saírem may be used when the speaker wants to emphasize who is leaving, or to avoid ambiguity.

Yes, but then the structure changes.

You can say:

• Antes de saírem... = before they leave
• Antes que saiam... = before they leave

These are both possible, but they use different grammar:

• antes de + infinitive / personal infinitive
• antes que + subjunctive

So:

• Antes de saírem, beijam-se...
• Antes que saiam, beijam-se...

The version with antes de saírem is very natural and elegant in Portuguese.

The comma separates the opening subordinate phrase from the main clause.

• Antes de saírem = introductory time clause
• beijam-se à porta de casa = main clause

So the comma helps mark the structure clearly:

• Before they leave, they kiss at the door.

It works much like English punctuation in similar sentences.

The accent shows that the i is pronounced separately from the a.

So saírem is pronounced roughly like:

• sa-EE-rem

The important point is that aí is not a single blended sound here. It breaks into separate syllables:

• sa-í-rem

That is why the written accent matters. It helps show the stress and syllable division.

Yes, absolutely.

Without more context, the sentence can describe:

• a general habit
• a typical routine
• a scene in narration
• a single event, depending on context

So it could mean something like:

• Before leaving, they usually kiss at the front door.

Portuguese present tense is often used this way, just like English simple present can be.