Às vezes, o bebé começa a berrar e o cão responde a latir, o que deixa toda a gente cansada.

Às vezes is a fixed expression meaning sometimes.

The word às is a contraction of a + as, and the grave accent marks that contraction. Literally, the expression is something like at times.

So:

• às vezes = sometimes
• às = contraction written with a grave accent

Portuguese uses definite articles much more often than English, especially before subject nouns.

So in a sentence like this, o bebé and o cão sound natural and normal:

• o bebé começa...
• o cão responde...

In English, we often drop the article in general statements, but Portuguese usually keeps it.

With bebé, the article can also reflect sex:

• o bebé = the baby boy, or sometimes a generic/default baby
• a bebé = the baby girl

This is a European Portuguese spelling.

• In Portugal: bebé
• In Brazil: bebê

They are the same word, but the spelling reflects pronunciation differences between the two varieties. Since this sentence is in Portuguese from Portugal, bebé is the expected form.

After começar, Portuguese normally uses a + infinitive to mean to start doing something.

So:

• começa a berrar = starts screaming / starts crying loudly

This is a very common structure:

• começar a falar = to start speaking
• começar a chover = to start raining
• começar a rir = to start laughing

So the a is not optional here in standard Portuguese.

Not exactly.

• chorar = to cry
• berrar = to yell, scream, shriek

When used about a baby, berrar suggests a louder, harsher, more intense kind of crying. It is more vivid than chorar.

So in this sentence, o bebé começa a berrar gives the idea that the baby starts crying very loudly.

Here responde a latir means responds by barking or reacts with barking.

It does not mean the dog is having a literal conversation. The idea is:

• the baby screams
• the dog reacts
• the dog’s reaction is barking

So:

• o cão responde a latir = the dog responds by barking

This is a natural way in Portuguese to describe a reaction expressed through an action.

They look the same, but the role is slightly different.

• In começa a berrar, a + infinitive is part of the normal pattern after começar
• In responde a latir, a + infinitive helps express how the dog responds

So both use a + infinitive, but the relationship with the main verb is not exactly identical.

A simple way to remember it is:

• começar a + infinitive = start to...
• responder a + infinitive = respond by...

In this sentence, o que refers to the whole previous situation, not just one noun.

So o que deixa toda a gente cansada means:

• which leaves everyone tired
• and that leaves everyone tired

What leaves everyone tired? Not just the baby, and not just the dog, but the whole sequence: the baby screaming and the dog barking in response.

This is a very common use of o que after a comma.

Because the grammatical subject of deixa is o que, and here o que is treated as singular.

Even though it refers to a whole situation involving several elements, Portuguese treats that whole event as one single thing:

• o que deixa... = which leaves...

So the verb stays singular:

• deixa, not deixam

Toda a gente means everyone or everybody.

Even though it refers to many people, it is grammatically singular in Portuguese. That is very important.

So you get:

• toda a gente está...
• toda a gente ficou...
• toda a gente cansada

Not:

• toda a gente estão...

It is a fixed, very common everyday expression.

Because cansada agrees with gente, and gente is grammatically feminine singular.

So:

• toda a gente cansada = literally something like all the people tired, but grammatically built around singular feminine gente

That is why the adjective is:

• cansada = feminine singular

If the sentence used todos instead, then you would expect:

• todos cansados

The comma shows that o que deixa toda a gente cansada is an added comment about the whole previous clause.

This kind of o que clause is non-restrictive: it does not identify a noun; it comments on the entire situation. Because of that, the comma is the normal punctuation.

Compare:

• O cão, que está no jardim, late muito.
  Here que adds extra information about o cão.
• O bebé começa a berrar..., o que deixa...
  Here o que comments on the whole event.

So the comma is both natural and important.