Marie s’arrête tout à coup.

Because the verb here is s’arrêter, a pronominal/reflexive verb. In this sentence, se becomes s’ before a vowel.

So:

• s’arrêter = to stop / to come to a stop
• Marie s’arrête = Marie stops

The s’ is not optional here. It is part of the verb as it is being used in this sentence.

Because arrêter without se usually means to stop something or to arrest someone.

For example:

• Marie arrête la voiture. = Marie stops the car.
• La police arrête le voleur. = The police arrest the thief.

But if Marie herself stops, French normally uses s’arrêter:

• Marie s’arrête. = Marie stops.

So Marie arrête by itself usually sounds incomplete unless something is being stopped is understood from context.

It is the present tense: specifically, the third-person singular present indicative of s’arrêter.

The subject is Marie, so French uses the same verb form it would use with il or elle:

• je m’arrête
• tu t’arrêtes
• il / elle / on s’arrête
• Marie s’arrête

In context, the French present can often match English stops, is stopping, or sometimes a vivid narrative present.

This is a spelling change that happens in many forms of the verb arrêter.

The infinitive is:

• arrêter

But in several present-tense forms, the é in the stem changes to è:

• j’arrête
• tu arrêtes
• il / elle arrête
• ils / elles arrêtent

This change helps show the pronunciation. In arrête, the vowel is pronounced like è, not like é.

Because se becomes s’ before a word beginning with a vowel or mute h.

So:

• se + arrête → s’arrête

This is very common in French:

• je me appelle becomes je m’appelle
• tu te arrêtes becomes tu t’arrêtes
• se arrêter becomes s’arrêter

It is simply a normal contraction to make pronunciation smoother.

Grammatically, tout à coup is an adverbial expression. It works like an adverb and tells you how or when the action happens.

In this sentence, it modifies s’arrête:

• Marie s’arrête tout à coup.

It means that the stopping happens suddenly or all at once.

Historically and literally, it can be connected with the idea of all at one blow or all at once, but for a learner it is best treated as a fixed idiom meaning:

• suddenly
• all of a sudden

So it is usually better not to translate it word by word every time. Learn tout à coup as one chunk.

Because in tout à coup, tout is part of a fixed expression. It does not change to match gender or number here.

So the expression is always:

• tout à coup

never:

• toute à coup
• tous à coup
• toutes à coup

This is one of those cases where the whole phrase should be memorized as a set expression.

No. In the present tense, the verb form does not change for masculine vs. feminine subjects.

So:

• Paul s’arrête.
• Marie s’arrête.

Both use the same verb form: s’arrête.

Gender can matter elsewhere in French, but not here in the present-tense verb ending.

A careful pronunciation is approximately:

ma-REE sa-RET too-ta-KOO

More precisely:

• Marie ≈ ma-ree
• s’arrête ≈ sa-ret
• tout à coup ≈ too-ta-koo

A few useful points:

• the s’ links directly to arrête
• the è in arrête is an open eh sound
• tout à coup is usually said smoothly as one expression

Yes. As an adverbial expression, it can sometimes be moved for emphasis.

For example:

• Marie s’arrête tout à coup.
• Tout à coup, Marie s’arrête.

Both are possible. The version with Tout à coup at the beginning can sound a little more dramatic or narrative, like storytelling.