Learn Hardcore French: Marie éteint son téléphone avant qu’elle s’endorme. - Marie turns off her phone before she falls asleep.

éteindre is the infinitive (to turn off), which you use after another verb (e.g. je vais éteindre, avant d’éteindre), not directly with a subject like Marie.
éteinte is the feminine past participle/adjective (turned off / switched off), used in forms like la lampe est éteinte (the lamp is off), not with a direct object like son téléphone in this structure.

Why is it son téléphone and not sa téléphone, even though Marie is female?

In French, possessive adjectives agree with the gender and number of the thing possessed, not with the owner.

téléphone is a masculine noun: un téléphone.
Therefore, the correct possessive is son (his/her/its + masculine singular noun), not sa.

So:

son téléphone = her phone (or his phone, depending on context; French does not mark the owner’s gender here)
If the noun were feminine, e.g. une lampe, we would say sa lampe.

Marie being female does not change son here; only téléphone’s gender matters.

What is the difference between avant que and avant de, and could we say avant de s’endormir here?

Yes, you could also say Marie éteint son téléphone avant de s’endormir.

The rule:

avant que is followed by a full clause (subject + verb in the subjunctive):
- avant que + sujet + verbe (subjonctif)
- … avant qu’elle s’endorme.
avant de is followed by an infinitive verb, used when the subject is the same in both parts:
- avant de + infinitif
- … avant de s’endormir. (subject is still Marie)

So in your sentence, both are grammatically possible:

Marie éteint son téléphone avant qu’elle s’endorme.
Marie éteint son téléphone avant de s’endormir.

The avant de + infinitive version is often a bit more neutral and slightly more common when the subject is the same.

Why is it s’endorme and not s’endort after avant que?

Because avant que normally requires the subjunctive mood in French.

s’endort is indicative present: a straightforward statement of fact.
s’endorme is subjunctive present: used after certain conjunctions like avant que, pour que, bien que, etc.

So:

Elle s’endort. → She is falling asleep / she falls asleep.
avant qu’elle s’endorme → before she falls asleep, with s’endorme in the subjunctive because of avant que.

The subjunctive here reflects that the action (falling asleep) is future/possible relative to éteint, not yet a completed fact at the time of speaking.

What tense or mood is s’endorme, and how is s’endormir conjugated?

S’endorme is third person singular, present subjunctive of s’endormir (to fall asleep).

Indicative present (normal present):

je m’endors
tu t’endors
il / elle s’endort
nous nous endormons
vous vous endormez
ils / elles s’endorment

Present subjunctive:

que je m’endorme
que tu t’endormes
qu’il / qu’elle s’endorme
que nous nous endormions
que vous vous endormiez
qu’ils / qu’elles s’endorment

In the sentence with avant que, we must use the subjunctive, so it is qu’elle s’endorme.

Why do we need the s’ in s’endorme? Can we just say endorme?

No. The s’ is essential here.

s’endormir is a pronominal reflexive verb, meaning to fall asleep.
Without the reflexive pronoun, endormir means to put someone to sleep (transitive):
- Je m’endors. = I fall asleep.
- J’endors le bébé. = I put the baby to sleep.

In elle s’endorme, the s’ (short for se) shows that she is the one who is falling asleep herself.

If you said avant qu’elle endorme, it would mean before she puts (someone) to sleep, which is a different meaning and sounds incomplete unless you add an object (e.g. avant qu’elle endorme le bébé).

What is qu’elle exactly, and why is there an apostrophe?

Qu’elle is simply que + elle.

que (that) ends with a vowel sound.
elle starts with a vowel sound.

French avoids having two vowel sounds together in many cases, so que becomes qu’ before a word starting with a vowel or mute h. This is called elision.

avant que elle s’endorme → becomes avant qu’elle s’endorme

The apostrophe marks the dropped -e.

The action of turning off the phone happens before she falls asleep in the future or as a habit. Why is French using the simple present éteint for that?

In French, the present tense is very commonly used for:

general habits / routines
near-future events, especially when another time expression is present or understood.

So Marie éteint son téléphone avant qu’elle s’endorme can mean:

A general habit: Marie always turns off her phone before she falls asleep.
A future situation in context: Tonight Marie turns off her phone before she falls asleep.

English often prefers the present simple or the future in this kind of sentence (turns off / will turn off), but French sticks to the present here without needing a will-future.

I have also seen avant qu’elle ne s’endorme with ne. Is something missing in this sentence?

Nothing is missing; your sentence is correct as it is.

avant qu’elle s’endorme
avant qu’elle ne s’endorme

Both are grammatically correct. The extra ne in the second version is called a ne explétif (pleonastic ne). It does not make the verb negative here; it’s stylistic and a bit more formal or literary.

Modern everyday French very often omits this ne, so your version without ne is perfectly normal in contemporary speech and writing.

Do we need a comma before avant qu’elle s’endorme?

No comma is required, and most writers do not put one here:

Marie éteint son téléphone avant qu’elle s’endorme.

In French, commas are used a bit differently than in English. A comma before subordinating conjunctions like avant que, parce que, quand is usually not obligatory and often omitted, unless you want a particular pause or emphasis.

Can we change the order and say Avant qu’elle s’endorme, Marie éteint son téléphone?

Yes, that is completely correct:

Avant qu’elle s’endorme, Marie éteint son téléphone.

Changing the order:

does not change the basic meaning,
slightly emphasizes the time condition (before she falls asleep) by putting it first.

Both orders are natural in French; it’s mainly a matter of style and rhythm.