Si la bebé se duerme temprano, mis amigas y yo podemos cenar tranquilas.

Because dormirse and dormir are not quite the same.

• dormir = to sleep
• dormirse = to fall asleep / to go to sleep

So la bebé se duerme temprano means the baby falls asleep early, not just that she sleeps early in general.

The se is part of the pronominal verb dormirse.

After si meaning if, Spanish normally uses the indicative, not the subjunctive, when talking about a real or possible condition in the present or future.

So:

• Si la bebé se duerme temprano... = If the baby falls asleep early...

Not:

• Si la bebé se duerma temprano... ❌

This is a very common rule:

• Si tengo tiempo, voy.
• Si llueve, nos quedamos en casa.
• Si la bebé se duerme temprano, podemos cenar tranquilas.

Spanish often uses the present tense to talk about a future situation, especially when it feels natural or likely from context.

So podemos cenar tranquilas here means something like:

• we can have dinner peacefully
• we’ll be able to have dinner peacefully

You could also say podremos cenar tranquilas, using the future tense, but podemos sounds very natural in everyday Spanish.

Because poder is a modal verb, and modal verbs in Spanish are followed by an infinitive.

So:

• podemos cenar = we can have dinner
• puedo ir = I can go
• pueden venir = they can come

This is like English can + verb, except Spanish uses the infinitive form.

Here tranquilas is an adjective describing the state of the people having dinner, not an adverb describing the action itself.

So cenar tranquilas means something like:

• to have dinner relaxed
• to have dinner in peace
• to have dinner without stress

It agrees with the understood subject, mis amigas y yo.

Because that group is feminine, the adjective is tranquilas.

Compare:

• mis amigas y yo podemos cenar tranquilas = we can have dinner relaxed / in peace
• mis amigos y yo podemos cenar tranquilos = same idea, but masculine/mixed group

Tranquilamente is possible in some contexts, but it would focus more on how the action is done, like calmly, rather than on the people’s state.

It agrees with mis amigas y yo.

The adjective must match the people it refers to:

• tranquila = feminine singular
• tranquilas = feminine plural
• tranquilo = masculine singular
• tranquilos = masculine plural or mixed group

Since the sentence says mis amigas y yo, and the adjective is tranquilas, that tells you the speaker is female too, so the whole group is feminine.

Yes. Bebé can be used with either masculine or feminine agreement depending on the baby’s sex:

• el bebé = the baby boy / the baby
• la bebé = the baby girl

In this sentence, la bebé shows that the baby is female.

You may also hear el bebé used more generally in some varieties of Spanish, but la bebé is completely normal when referring to a girl.

Because the stress falls on the last syllable: be-BÉ.

Without the accent, Spanish spelling rules would make you stress it differently. The written accent shows the correct pronunciation.

So:

• bebé = stressed on the last syllable

Here temprano is being used as an adverb, meaning early, so it does not agree with bebé.

• se duerme temprano = she falls asleep early

Compare that with adjective use:

• una hora temprana = an early hour

In your sentence, it is not describing the baby; it is describing when she falls asleep.

Because Spanish often leaves subject pronouns out when they are not needed.

Instead of saying:

• Nosotras podemos cenar tranquilas

the sentence says:

• mis amigas y yo podemos cenar tranquilas

That already tells you who the subject is, so nosotras would be unnecessary.

Spanish usually only includes subject pronouns for emphasis, contrast, or clarity.

In Spanish, as in English, it is more natural and polite to put yourself after the other people:

• mis amigas y yo = my friends and I

Saying yo y mis amigas is grammatically understandable, but it sounds less natural and can seem self-centered.

So mis amigas y yo is the normal order.

Because the sentence begins with an if-clause:

• Si la bebé se duerme temprano, ...

In Spanish, when a subordinate clause like this comes first, it is normal to separate it with a comma from the main clause.

So:

• Si la bebé se duerme temprano, mis amigas y yo podemos cenar tranquilas.

If the main clause came first, the comma would usually be omitted:

• Mis amigas y yo podemos cenar tranquilas si la bebé se duerme temprano.