Pendant son exposé, Marie utilise un surligneur jaune pour montrer les idées principales.

Here pendant means during.

So Pendant son exposé means During her presentation.

A useful detail: pendant can also mean for with a length of time, as in J’ai attendu pendant deux heures = I waited for two hours. In this sentence, though, it clearly means during.

Because son / sa / ses agree with the thing possessed, not with the owner.

• Marie is female, but that does not control the choice.
• exposé is a masculine singular noun.
• So you use son.

Compare:

• son exposé = her presentation because exposé is masculine
• sa voiture = her car because voiture is feminine

This is very common in French and often surprises English speakers.

In French, un exposé usually means a presentation, a talk, or sometimes a report given orally, especially in school or formal contexts.

It is not usually the same as the English noun exposé, which often means a shocking report that reveals hidden facts.

So in French:

• faire un exposé = to give a presentation
• un exposé en classe = a class presentation

Utilise is the third-person singular present form of utiliser.

The sentence is describing what Marie does, so French uses the present tense:

• Marie utilise = Marie uses / is using

Just like in English, the simple present can describe:

• a habitual action
• a general description
• something happening in a narrated situation

Here it works as a straightforward descriptive present.

Because the subject is Marie, which is third-person singular, and utilise is the matching verb form.

The infinitive is utiliser = to use.

Present tense:

• j’utilise = I use
• tu utilises = you use
• il / elle utilise = he / she uses

So Marie utilise means Marie uses.

Un surligneur means a highlighter.

It comes from the verb surligner, which means to highlight.

So:

• un surligneur = a highlighter
• surligner un texte = to highlight a text

In everyday French, this is the normal word for the pen/marker used to highlight text.

Because in French, most color adjectives come after the noun.

So:

• un surligneur jaune = a yellow highlighter
• une voiture rouge = a red car
• un livre bleu = a blue book

English usually puts color adjectives before the noun, but French usually puts them after.

Un is the indefinite article, meaning a or an.

So un surligneur jaune means a yellow highlighter.

French usually requires an article where English does too, and often even where English might omit one. Here, it is simply introducing one non-specific object:

• un surligneur = a highlighter

Pour montrer means in order to show or simply to show.

This is pour + infinitive, which often expresses purpose.

So the structure is:

• Marie utilise un surligneur jaune = Marie uses a yellow highlighter
• pour montrer les idées principales = to show the main ideas

Together: she uses the highlighter for the purpose of showing the main ideas.

This is a very common French pattern:

• Il travaille pour gagner de l’argent = He works to earn money
• Je viens pour aider = I’m coming to help

French generally uses articles more often than English.

Here les idées principales means the main ideas.

The definite article les is needed because we are talking about a specific set of ideas in the presentation or text. In French, leaving out the article here would sound incomplete or unnatural.

So:

• les idées principales = the main ideas

Because adjectives in French agree with the noun they describe.

Here:

• idées is feminine plural
• so principal must become principales

Agreement pattern:

• principal = masculine singular
• principale = feminine singular
• principaux = masculine plural
• principales = feminine plural

Since idées is feminine plural, the correct form is principales.

Yes, adjectives normally agree with the noun they describe, but jaune happens to look the same in masculine and feminine singular.

Here:

• surligneur is masculine singular
• so jaune stays jaune

Examples:

• un surligneur jaune
• une feuille jaune

In the plural, you would add -s:

• des surligneurs jaunes
• des feuilles jaunes

Yes, in this sentence durant would be possible.

• Pendant son exposé = During her presentation
• Durant son exposé = During her presentation

Both are correct. In many contexts they are very close in meaning. Pendant is often a little more common in everyday French, while durant can sound slightly more formal depending on context.

A few useful points:

• pendant is roughly pronounced pahn-dahn
• son sounds like sohn with a nasal vowel
• exposé ends with an ay sound: eks-po-zay
• utilise sounds roughly like yoo-tee-leez
• surligneur can be tricky because of gn, which sounds like ny in canyon
• idées has three syllables: ee-day
• principales is roughly pran-see-pal

Also, in natural speech, French rhythm connects words smoothly, so Pendant son exposé flows together rather than being said word by word.