Pour son exposé de demain, Marie relit chaque fiche lentement afin de ne rien oublier.

This opening phrase gives the context/purpose first: for her presentation tomorrow.

French often puts this kind of phrase at the beginning to set the scene before giving the main action:

• Pour son exposé de demain, Marie... = For her presentation tomorrow, Marie...
• It could also come later:
  • Marie relit chaque fiche lentement pour son exposé de demain.

Both are grammatical, but putting it first makes the presentation feel like the main context for everything that follows.

Here, pour means for.

So:

• Pour son exposé de demain = For her presentation tomorrow

It introduces the purpose or intended use of what follows. Marie is rereading the cards for that presentation.

Be careful: later in the sentence, French uses afin de to express another kind of purpose:

• afin de ne rien oublier = so as not to forget anything

So this sentence contains two purpose expressions:

• pour son exposé de demain = for her presentation tomorrow
• afin de ne rien oublier = in order not to forget anything

Because exposé is a masculine singular noun.

French possessive adjectives agree with the noun possessed, not with the owner.

So:

• son exposé = her presentation
• not because Marie is male, but because exposé is masculine

Compare:

• son exposé = her presentation
• sa fiche = her card/sheet

So the choice between son / sa / ses depends on the noun after it, not on the person who owns it.

Here, un exposé means a presentation, report, or sometimes a talk given in class or in a formal setting.

For a learner of French, it is useful to know that it often refers to a school or academic presentation.

So:

• son exposé de demain = her presentation tomorrow / her presentation for tomorrow

French usually expresses this idea with de + time word.

So:

• l’exposé de demain = tomorrow’s presentation / the presentation for tomorrow

This is very common in French:

• le cours de demain = tomorrow’s class
• la réunion de lundi = Monday’s meeting

English can use the possessive-like form tomorrow’s presentation, but French normally uses de instead.

Relit is the present tense of the verb relire, which means to reread.

So:

• Marie relit = Marie rereads / Marie is rereading

This verb is built from:

• lire = to read
• relire = to read again, reread

It is an irregular verb, because it follows the pattern of lire.

Present tense:

• je relis
• tu relis
• il/elle relit
• nous relisons
• vous relisez
• ils/elles relisent

So relit is the third-person singular form.

Because chaque means each, and in French it is followed by a singular noun.

So:

• chaque fiche = each card / each sheet

Even though the idea refers to multiple items one by one, French uses the singular after chaque.

Compare:

• chaque fiche = each card
• toutes les fiches = all the cards

Fiche can mean different things depending on context, but here it most likely means a note card, index card, or sheet of notes.

In a study or presentation context, une fiche often refers to:

• a revision card
• a cue card
• a summary sheet

So Marie relit chaque fiche suggests she is going through her notes one by one.

Because lentement is an adverb, and in French adverbs often come after the verb or after the verb’s object, depending on style and emphasis.

Here:

• Marie relit chaque fiche lentement

means:

• Marie rereads each card slowly

This word order is very natural in French.

You could sometimes see an adverb placed differently, but this version is straightforward and idiomatic. The adverb describes how she rereads the cards.

Both can express purpose, but afin de is usually a bit more formal than pour.

In this sentence:

• Pour son exposé de demain = for her presentation tomorrow
• afin de ne rien oublier = so as not to forget anything / in order not to forget anything

So they are not doing exactly the same job:

• pour introduces the general context or intended use
• afin de introduces the goal of the action

In everyday spoken French, many people would simply say:

• ... pour ne rien oublier

instead of:

• ... afin de ne rien oublier

Both are correct, but afin de sounds a little more formal or polished.

This is an infinitive construction after afin de:

• afin de + infinitive = in order to + verb

The verb here is:

• oublier = to forget

Negation with an infinitive works like this:

• ne pas oublier = not to forget
• ne rien oublier = not to forget anything

So:

• afin de ne rien oublier = in order not to forget anything

Notice the word order:

• ne comes before the infinitive
• rien comes after it in this structure

That is normal in formal written French.

Because French commonly expresses nothing / not anything with the negative structure ne ... rien.

So:

• ne rien oublier literally works like to not forget anything
• natural English: not to forget anything

French does not usually say a simple positive form equivalent to forget nothing in ordinary usage. The standard structure is the negative one:

• ne ... rien = not ... anything / nothing

Examples:

• Je ne vois rien. = I see nothing / I don’t see anything.
• Elle ne dit rien. = She says nothing / She doesn’t say anything.

Yes. The French present tense can often translate in more than one way in English, depending on context.

So:

• Marie relit chaque fiche lentement could be:
  • Marie rereads each card slowly
  • Marie is rereading each card slowly

In this sentence, because it describes what she is doing for tomorrow’s presentation, English often prefers the progressive form:

• Marie is rereading each card slowly...

But the French itself is simply the present tense.

It sounds mostly neutral to slightly formal, especially because of afin de.

The more formal-feeling part is:

• afin de ne rien oublier

In more everyday spoken French, someone might say:

• Pour son exposé de demain, Marie relit chaque fiche lentement pour ne rien oublier.

That version is a bit more conversational. The original sentence is still perfectly natural, especially in written French.