Paul marque les verres avec un feutre pour ne pas les confondre.

Here marque comes from marquer, which means to mark, to label, or to make a sign on something.

So Paul marque les verres means Paul is putting some kind of mark on the glasses, not scoring or making a brand in the English sense.

It is in the present tense:

• je marque
• tu marques
• il/elle marque

So Paul marque = Paul marks / is marking.

Les verres means the glasses, while des verres means some glasses.

French often uses the definite article when talking about a specific set of things that is already understood from the situation. Here, Paul is marking a particular group of glasses so people do not mix them up.

So:

• les verres = the glasses, those specific glasses
• des verres = some glasses, an unspecified number of glasses

In this sentence, les verres sounds natural because these are the glasses involved in the situation.

Here verres means drinking glasses.

French un verre can mean:

• a glass as a drinking container
• a glassful / drink in some contexts
• glass as a material in other contexts

It does not normally mean eyeglasses in this kind of sentence. Eyeglasses are usually:

• des lunettes

So here les verres = drinking glasses.

Avec un feutre means with a felt-tip pen / marker.

The word feutre can mean felt, but in everyday French un feutre often means a felt-tip pen or marker.

So:

• avec = with
• un feutre = a marker / felt-tip pen

In natural English, the whole phrase is usually understood as with a marker.

Un feutre means a marker, not a specific one already identified.

French uses:

• un / une for a / an
• le / la / les for the

So avec un feutre means Paul is using a marker, any marker, not necessarily one that has already been mentioned.

If it were avec le feutre, it would mean with the marker, implying a specific marker known to the speaker and listener.

Here pour means to, in order to, or so as to.

It introduces the purpose of the action:

• Paul marque les verres = Paul marks the glasses
• pour ne pas les confondre = so as not to mix them up

This is a very common French structure:

pour + infinitive

• Je travaille pour gagner de l’argent. = I work to earn money.
• Il part tôt pour éviter le trafic. = He leaves early to avoid traffic.

So in your sentence, pour explains why Paul marks the glasses.

After pour, French normally uses an infinitive when the subject stays the same.

Here the subject of both actions is Paul:

• Paul marks the glasses
• Paul does not want to mix them up

So French uses:

• pour ne pas les confondre

This is more natural than using a full clause.

Compare:

• Il écrit son nom pour ne pas oublier son livre. = He writes his name so as not to forget his book.
• Elle prend des notes pour mieux comprendre. = She takes notes to understand better.

If the subject changed, French would often use a different structure, such as pour que + subjunctive.

Because confondre is an infinitive, not a fully conjugated verb.

In French, when you negate an infinitive, ne pas goes directly before the infinitive:

• ne pas manger = not to eat
• ne pas oublier = not to forget
• ne pas confondre = not to confuse / not to mix up

There is no separate subject inside that infinitive phrase because the subject is already understood from the main clause: it is still Paul.

So:

• Paul marque les verres pour ne pas les confondre. Literally: Paul marks the glasses in order not to mix them up.

The les refers back to les verres.

It is a direct object pronoun meaning them.

So instead of repeating les verres, French replaces it with les:

• confondre les verres = to mix up the glasses
• les confondre = to mix them up

French object pronouns usually come before the verb or infinitive:

• Je les vois. = I see them.
• pour les confondre = to mix them up
• pour ne pas les confondre = so as not to mix them up

Yes, that is grammatically possible.

Both are understandable:

• pour ne pas les confondre
• pour ne pas confondre les verres

But pour ne pas les confondre sounds more natural here because les verres has just been mentioned, so French prefers the pronoun instead of repeating the noun.

Using the pronoun avoids repetition, just like English often prefers them instead of repeating the glasses.

Confondre means to confuse, to mistake one thing for another, or to mix up.

In this sentence, the most natural English translation is to mix them up.

Examples:

• Je confonds toujours ces deux mots. = I always mix up these two words.
• Ne me confonds pas avec mon frère. = Don’t mistake me for my brother.

So here, ne pas les confondre means not to mix up the glasses.

Because French often expresses the same idea with a single verb where English uses a verb + particle.

English:

• mix up
• pick up
• turn on

French often uses one verb:

• confondre = mix up / confuse
• ramasser = pick up
• allumer = turn on

So confondre already contains the idea that English expresses with mix ... up.

Yes, it is natural.

The structure is:

• marquer quelque chose = to mark something
• avec
  • tool/instrument = with + tool

So:

• marquer les verres avec un feutre = to mark the glasses with a marker

French uses avec to show the instrument:

• écrire avec un stylo = to write with a pen
• couper avec un couteau = to cut with a knife
• dessiner avec un crayon = to draw with a pencil

The final s of les and verres is normally not pronounced on its own.

But there is a liaison between les and verres:

• les verres sounds roughly like lay vair

More precisely:

• les = /le/
• before a vowel sound, the normally silent s in les is pronounced like z
• verres begins with a vowel sound, so you get les‿verres = /lez vɛʁ/

The final s of verres is still silent.

Because the sentence does not necessarily mean the glasses belong to Paul.

Les verres just means the glasses. They could be:

• Paul’s glasses
• several people’s glasses
• glasses at a party
• glasses on a table

French only adds a possessive like ses verres if ownership is important:

• Paul marque ses verres = Paul marks his glasses

Here the sentence is about identifying the glasses so they do not get mixed up, not about who owns them, so les verres is enough.