Se a bebê chorar de novo, a mãe vai pegá-la no colo.

Because the sentence is talking about a female baby. In Brazilian Portuguese, bebê often keeps the same noun form for both sexes, and the article shows the gender:

• o bebê = the baby boy / the baby (masculine or generic)
• a bebê = the baby girl

So a bebê tells you the baby is female.

After se when you are talking about a possible future situation, Portuguese normally uses the future subjunctive.

So:

• Se a bebê chorar de novo... = If the baby cries again...

This is the normal pattern for real future conditions in Portuguese.

A native English speaker may expect something like a present tense here, but Portuguese uses the future subjunctive instead.

It is the future subjunctive, not the infinitive, even though it looks exactly the same.

For many regular verbs, the future subjunctive has the same form as the infinitive:

• chorar = to cry
• se ela chorar = if she cries

You can see the difference more clearly with irregular verbs:

• ir = to go
• se ela for = if she goes

So in your sentence, chorar is a conjugated verb form.

De novo means again.

So:

• chorar de novo = cry again

This is a very common everyday way to say again in Brazilian Portuguese. Another option is novamente, but de novo sounds more natural in casual speech.

Because ir + infinitive is extremely common in Brazilian Portuguese for the future.

So:

• vai pegá-la = will pick her up

This is usually more natural in everyday Brazilian speech than the simple future:

• pegará

Both are grammatically possible, but vai + infinitive is much more common in conversation.

Here, pegar means to pick up.

By itself, pegar can mean several things depending on context, such as:

• to take
• to grab
• to catch
• to pick up

In this sentence, because of no colo, it clearly means pick the baby up into her arms / hold the baby.

Because the direct object pronoun is attached to the verb.

The underlying pieces are:

• pegar = to pick up
• a = her

When a verb ending in -r, -s, or -z takes the clitic pronoun o / a / os / as, the ending changes:

• the final -r drops
• a becomes -la

So:

• pegar + a → pegá-la

The accent is added to show the correct stress after the change.

Because -la is the unstressed direct object pronoun attached to the verb.

In standard grammar:

• ela = she / her as a stressed pronoun
• a / -la = her as a direct object clitic

So:

• vai pegá-la = will pick her up

Using ela directly after the verb is very common in spoken Brazilian Portuguese:

• vai pegar ela

But in more formal or standard written Portuguese, vai pegá-la is preferred.

No colo means something like in (someone’s) arms or on (someone’s) lap, depending on context.

Literally:

• no = em + o
• colo = lap / the area of the arms and chest where you hold a child

With babies, pegar no colo usually means:

• to take the baby into your arms
• to hold the baby

So the full idea is that the mother will pick the baby up and hold her.

Because Portuguese often uses the definite article when the relationship is already clear from context.

So:

• a mãe = the mother

In this sentence, it is understood to mean the baby’s mother. Portuguese does not always need to say her mother or the baby’s mother if that is already obvious.

This is very natural in Portuguese, even though English often prefers a possessive like her mother in the same situation.

Yes, but mostly in more careful speech or writing.

In everyday spoken Brazilian Portuguese, many people would more naturally say:

• Se a bebê chorar de novo, a mãe vai pegar ela no colo.

That is very common in speech.

The version in your sentence:

• vai pegá-la

is more standard and more formal-sounding. So it is correct and important to learn, but you should know that casual spoken Brazilian Portuguese often uses pegar ela instead.