À moins que le bébé dorme déjà, ma sœur enlèvera le siège auto avant de vider le coffre.

À moins que introduces a full clause with its own subject and verb: le bébé dorme déjà.

• À moins que + clause = unless
• À moins de + infinitive/noun = unless / except by / without, but with no new conjugated verb

So here we need que because we are saying unless the baby is already asleep, which contains a subject (le bébé) and a verb (dorme).

Compare:

• À moins que le bébé dorme déjà... = Unless the baby is already asleep...
• À moins de partir maintenant... = Unless we leave now / except by leaving now...

Because à moins que normally requires the subjunctive.

So:

• indicative: le bébé dort
• subjunctive: le bébé dorme

Many conjunctions in French trigger the subjunctive, especially when they express uncertainty, condition, emotion, purpose, or possibility. À moins que is one of them.

That is why the sentence uses:

• À moins que le bébé dorme déjà

and not:

• À moins que le bébé dort déjà

Often, yes. In more formal or traditional French, you will frequently see:

• À moins que le bébé ne dorme déjà

This ne is called an expletive ne. It does not make the sentence negative. It is just a stylistic/grammatical feature that appears after certain expressions such as à moins que, avant que, de peur que, and some others.

So both of these can be understood as:

• À moins que le bébé dorme déjà
• À moins que le bébé ne dorme déjà

The version without the extra ne is common, especially in everyday spoken French.

Because the sentence is talking about an action that will happen later.

• enlever = to remove
• elle enlèvera = she will remove

The structure is:

• condition/exceptions first: À moins que le bébé dorme déjà
• main future action second: ma sœur enlèvera le siège auto

French uses the simple future here just as English often uses will:

• My sister will remove the car seat...

Because the subject is the same for both actions: ma sœur.

French usually uses:

• avant de + infinitive when the subject stays the same

So here, ma sœur is the person who will:

1. remove the car seat
2. empty the trunk

That is why French says:

• ma sœur enlèvera le siège auto avant de vider le coffre

If the subject changed, French would use avant que + subjunctive:

• Ma sœur enlèvera le siège auto avant que mon père ne vide le coffre.

So:

• same subject -> avant de + infinitive
• different subject -> avant que + subjunctive

Because it follows avant de.

The pattern is:

• avant de + infinitive

Examples:

• avant de partir = before leaving
• avant de manger = before eating
• avant de vider le coffre = before emptying the trunk

This is very common in French when one subject does two actions.

Here déjà means already.

In French, short adverbs like déjà, souvent, bien, toujours often come after the conjugated verb, especially in simple tenses.

So:

• le bébé dort déjà
• le bébé dorme déjà

This is the normal placement.

In English, already can move around a little more, but French is usually more fixed here.

Siège auto is a very natural and common term for a car seat, especially a child’s car seat.

A few related expressions:

• siège auto = car seat, often specifically a child car seat
• siège bébé = baby seat; understandable, but more context-dependent
• siège de voiture = literally car seat, but this can also mean an ordinary seat in a car, not necessarily a child safety seat

So in this sentence, le siège auto is a good choice because it clearly suggests the safety seat for the baby.

Not necessarily. Bébé is grammatically masculine, so it takes le by default:

• le bébé

But grammatical gender and real-life sex are not always the same thing. A girl baby can still be referred to as le bébé.

If you wanted to make the baby’s sex explicit, you could say:

• la petite fille
• le petit garçon

But le bébé is neutral in meaning even though it is grammatically masculine.

Because vider directly takes an object.

• vider quelque chose = to empty something

So:

• vider le coffre = to empty the trunk

You would use dans le coffre if you were talking about location instead:

• mettre les sacs dans le coffre = to put the bags in the trunk

But with vider, the trunk itself is the direct object, so le coffre is correct.

Yes, it is completely natural to start with the à moins que clause.

French can place this kind of clause either before or after the main clause:

• À moins que le bébé dorme déjà, ma sœur enlèvera le siège auto...
• Ma sœur enlèvera le siège auto..., à moins que le bébé dorme déjà.

Starting with it gives the exception or condition first, which is often useful for emphasis or clarity.

Both are grammatical; the choice is mostly about style and focus.