Même si le bébé pleure, la nounou reste calme et lui met le bavoir avant de préparer le biberon.

Même si means even if or even though.

In this sentence, it introduces a contrast:

• Même si le bébé pleure = Even though the baby is crying / cries

It shows that the second part is true despite the first part:

• the baby is crying,
• but the nanny still stays calm.

For English speakers, même si is a very common way to say even if / even though before a full clause with a verb.

French often uses the simple present where English might use either the simple present or the present continuous.

• le bébé pleure literally: the baby cries
• but in context it often means: the baby is crying

So Même si le bébé pleure is a very natural French way to say Even if/though the baby is crying.

French does have être en train de + infinitive, but it is used only when you really want to emphasize that the action is in progress:

• Le bébé est en train de pleurer = The baby is in the middle of crying / is crying right now

Here, the simple form pleure is enough.

La nounou is an informal, everyday word for a person who looks after children. Depending on context, it can mean:

• nanny
• childminder
• babysitter

It is very common in spoken French and feels warm and natural.

A more formal term might be:

• la nourrice (older or more traditional in some contexts)
• la baby-sitter (for a babysitter specifically)
• l’assistante maternelle (official/professional term in France)

In this sentence, la nounou is the most natural word if you mean the person caring for the baby.

Both are possible in French, but they do not mean exactly the same thing.

• elle est calme = she is calm
  This describes her state.
• elle reste calme = she stays/remains calm
  This emphasizes that she continues to be calm despite the crying.

Because the sentence begins with Même si le bébé pleure, reste calme is especially good: it highlights that the nanny does not lose her calm.

So:

• la nounou reste calme = the nanny stays calm

Calme is an adjective with the same form in both masculine and feminine singular.

So you get:

• un homme calme
• une femme calme

No spelling change is needed.

Some adjectives do change:

• petit → petite
• fatigué → fatiguée

But calme stays calme.

Here, lui means to him or to her. In this sentence, it refers to le bébé.

So:

• lui met le bavoir = puts the bib on him/her

More literally:

• mettre quelque chose à quelqu’un = to put something on someone
• elle lui met le bavoir = she puts the bib on him/her

This lui is an indirect object pronoun.

Because the verb pattern here is indirect, not direct.

In French:

• mettre quelque chose à quelqu’un
  = to put something on someone

So the thing being put is the direct object:

• le bavoir

And the person receiving it is the indirect object:

• au bébé → lui

Compare:

• Elle met le bavoir au bébé.
• Elle lui met le bavoir.

If you used le or la, that would mean the person is the direct object, which is not how mettre le bavoir à quelqu’un works.

In French, object pronouns usually come before the conjugated verb.

So:

• Elle lui met le bavoir.
• not Elle met lui le bavoir.

This is one of the most important French pronoun rules.

A few examples:

• Je lui parle = I’m talking to him/her
• Elle le voit = She sees him/it
• Nous leur donnons le livre = We give them the book

So in your sentence:

• la nounou ... lui met le bavoir = the nanny ... puts the bib on him/her

Le bavoir means the bib.

It is the cloth or plastic item put around a baby’s neck during feeding to protect clothes.

This is a very common baby-related word in French.

French often uses the definite article (le, la, les) where English might say a or the, depending on context.

Here, le bavoir and le biberon refer to the expected, familiar items involved in caring for the baby:

• the bib
• the baby bottle

Using le sounds natural because these are specific items in the situation, even if they were not mentioned earlier.

If you said un bavoir or un biberon, it would sound more like introducing one item among others:

• Elle lui met un bavoir = She puts a bib on him/her
• Elle prépare un biberon = She prepares a bottle

That is possible, but in this sentence le feels very natural.

Because when avant is followed by a verb in the infinitive, French uses de:

• avant de + infinitive

So:

• avant de préparer le biberon = before preparing the bottle / before she prepares the bottle

This is a fixed pattern.

Examples:

• avant de partir = before leaving
• avant de manger = before eating
• avant de dormir = before sleeping

If a full clause follows instead, French uses avant que:

• avant qu’elle prépare le biberon = before she prepares the bottle

But in your sentence, the infinitive is used, so it must be avant de préparer.

Because after avant de, French uses the infinitive when the subject is understood or the same.

So:

• la nounou ... avant de préparer le biberon
• literally: the nanny ... before preparing the bottle

The subject of préparer is understood to be the nanny.

This is similar to English:

• She stays calm before preparing the bottle.

If you wanted a separate clause with an explicit subject, French would use:

• avant qu’elle prépare le biberon

But that is not necessary here.

Préparer le biberon means to prepare the baby bottle.

In real-life context, that usually means getting the bottle ready for feeding, for example:

• making up the milk
• warming it if needed
• checking it is ready for the baby

So le biberon here often means not just the object itself, but the bottle as prepared for the feed.

French does not need to repeat the subject when the same subject continues across coordinated verbs.

So:

• la nounou reste calme et lui met le bavoir
• literally: the nanny stays calm and puts the bib on him/her

French understands that la nounou is still the subject of met.

The same is true with the infinitive phrase:

• avant de préparer le biberon
• the understood subject is still la nounou

English works similarly:

• The nanny stays calm and puts the bib on him before preparing the bottle.

A natural English translation could be:

• Even though the baby is crying, the nanny stays calm and puts the bib on him before preparing the bottle.

Depending on context, you could also say:

• Even if the baby cries, the nanny remains calm and puts his bib on before preparing the bottle.

The exact English choice depends on whether you hear pleure as a general situation (cries) or something happening now (is crying). In French, the same present tense form can cover both.