Quando a bebê está com sono, ela fica nervosa.

Because bebê can refer to either a male or a female baby. In Brazilian Portuguese, the article shows the gender:

• o bebê = the baby boy / the baby (male)
• a bebê = the baby girl / the baby (female)

The adjective later in the sentence also agrees with that gender:

• a bebê ... nervosa
• o bebê ... nervoso

So here, a bebê tells you the baby is female.

Here quando is best understood as when or whenever, depending on context.

In this sentence, it has a general meaning:

• Quando a bebê está com sono, ela fica nervosa.
• When/Whenever the baby is sleepy, she gets cranky/agitated.

Because the rest of the sentence is in the present tense, it sounds like a habitual or repeated situation, not just one single event.

Estar com sono is a very common and natural way to say to be sleepy or to feel sleepy in Portuguese.

Literally, com sono means with sleep, but idiomatically it means:

• estar com sono = to be sleepy / to feel sleepy

You can also say sonolenta for a female:

• A bebê está sonolenta.

But estar com sono is often more everyday and conversational.

Yes, ter sono exists and is understandable:

• A bebê tem sono.

But in Brazilian Portuguese, estar com sono is usually more common when talking about someone feeling sleepy right now.

A rough comparison:

• está com sono = she is sleepy right now
• tem sono = she has sleepiness / she tends to feel sleepy

In many situations, both work, but está com sono sounds especially natural here.

Because ficar often means to become or to get.

So:

• ela fica nervosa = she gets nervous / agitated / cranky

This suggests a change of state. The baby starts out sleepy, and then as a result, she becomes upset.

Compare:

• ela está nervosa = she is nervous/upset
• ela fica nervosa = she gets nervous/upset

Here, fica fits very well because it describes what happens when she is sleepy.

Not exactly in the English sense of anxious. In Brazilian Portuguese, nervoso/nervosa can mean:

• nervous
• upset
• agitated
• irritated
• cranky

With a baby, fica nervosa usually means something like:

• gets fussy
• gets upset
• gets cranky
• gets agitated

So this is a good example of a word that does not always match English one-to-one.

Because it agrees with a bebê, which is feminine in this sentence.

In Portuguese, adjectives usually agree in gender and number with the noun they describe:

• a bebê nervosa
• o bebê nervoso

So the ending -a in nervosa matches the feminine noun phrase a bebê.

Yes, you often can omit ela in Portuguese:

• Quando a bebê está com sono, fica nervosa.

That is grammatically fine and sounds natural.

However, including ela can make the sentence a little clearer or more explicit:

• Quando a bebê está com sono, ela fica nervosa.

Portuguese often drops subject pronouns, but speakers may still include them for clarity, emphasis, or rhythm.

The comma separates the introductory clause from the main clause:

• Quando a bebê está com sono, = when the baby is sleepy
• ela fica nervosa. = she gets upset/cranky

This is very similar to English:

• When the baby is sleepy, she gets cranky.

The comma is standard and helps readability.

Because the present tense in Portuguese, just like in English, can describe habitual or repeated situations.

So:

• Quando a bebê está com sono, ela fica nervosa.

means something like:

• whenever she is sleepy, she gets cranky

This is not limited to one moment. It describes a pattern.

Yes, but the meaning shifts slightly.

• Quando a bebê está com sono, ela fica nervosa.
  = This is a general fact or usual pattern.

• Quando a bebê estiver com sono, ela ficará nervosa / vai ficar nervosa.
  = This refers to a future occasion: when the baby becomes sleepy, she will get upset.

So the original sentence uses the present because it describes a general truth or repeated behavior.

Yes, to some extent. Portuguese allows some variation.

For example, these are all possible:

• Quando a bebê está com sono, ela fica nervosa.
• A bebê fica nervosa quando está com sono.
• Quando está com sono, a bebê fica nervosa.

The original version is very natural because it sets up the condition first. That makes it easy to understand: first the sleepy state, then the result.

Bebê is stressed on the last syllable because of the accent:

• be-BÊ

The final ê/é sound is clearly stressed. The acute accent tells you where the stress goes.

So it sounds roughly like:

• beh-BEH

with the second syllable stronger than the first.

People use several words for baby, depending on region, tone, and family habits. Common ones include:

• bebê
• neném
• criança (less specifically child)
• bebezinha / bebezinho for a more affectionate tone

So you might also hear:

• Quando o neném está com sono, ele fica nervoso.

But a bebê is completely natural and standard.