Marie rit en voyant la mousse sur le sol, puis elle finit par l’essuyer.

Because the verb is rire (to laugh), and its third-person singular present form is rit:

• je ris
• tu ris
• il/elle rit
• nous rions
• vous riez
• ils/elles rient

So Marie rit means Marie laughs or Marie is laughing.

This is a common point of confusion because many -re verbs have unusual forms, and rit is simply the correct irregular form of rire.

En voyant means something like on seeing, when she sees, or while seeing, depending on context.

This is the structure:

• en + present participle

Here:

• voir = to see
• voyant = seeing
• en voyant = upon/while seeing

So Marie rit en voyant la mousse sur le sol means she laughs when / on seeing the foam on the floor.

This structure often expresses:

• something happening at the same time
• or something happening because of what is seen/done

Examples:

• Il est parti en souriant. = He left smiling.
• Elle a appris la nouvelle en lisant le journal. = She learned the news while reading the newspaper.

Because after en, French normally uses the present participle, not the infinitive.

So:

• voir = infinitive = to see
• voyant = present participle = seeing

That is why French says:

• en voyant = on seeing / while seeing

and not:

• en voir

La mousse most often means foam, froth, or suds.

Depending on context, it can also mean other things, such as:

• moss
• mousse (the dessert)
• foam material

But in this sentence, because it is sur le sol (on the floor) and later elle finit par l’essuyer (she ends up wiping it), the most natural meaning is foam/suds on the floor.

French often uses an article where English might not.

So French says:

• la mousse
• le sol
• le café
• la pluie

even in places where English might say:

• foam
• floor
• coffee
• rain

In this sentence, la mousse refers to a specific visible foam that Marie sees, so the definite article la feels natural.

Both can often mean on the floor / on the ground, but there is a nuance.

• sur le sol = literally on the floor/on the ground
• par terre = on the floor, often in a more everyday, idiomatic way

Sur le sol can sound a little more neutral, descriptive, or formal. Par terre is very common in spoken French.

So this sentence could also have been:

• Marie rit en voyant la mousse par terre...

That would sound slightly more conversational.

Puis means then.

It links one action to the next:

• first, Marie laughs
• then, she ends up wiping it

It is similar to:

• ensuite = next / afterwards
• puis = then

In many contexts, puis sounds a bit smoother or more narrative.

Finir par + infinitive is a very common French expression meaning:

• to end up doing something
• to finally do something
• to eventually do something

So:

• elle essuie la mousse = she wipes the foam
• elle finit par l’essuyer = she ends up wiping it / she finally wipes it

The second version adds the idea that:

• maybe she did not want to do it at first
• maybe there was hesitation
• but in the end, she did it

So finit par adds an important nuance.

Because finir par is followed by an infinitive.

The pattern is:

• finir par + infinitive

Examples:

• Il finit par accepter. = He ends up accepting.
• Nous finissons par comprendre. = We eventually understand.
• Elle finit par l’essuyer. = She ends up wiping it.

So:

• essuyer = correct, because it is the infinitive
• essuie = wrong here, because that is a conjugated form
• essuyant = wrong here, because that is a present participle

The l’ is a direct object pronoun meaning it.

It replaces la mousse.

So instead of repeating:

• elle finit par essuyer la mousse

French uses:

• elle finit par l’essuyer

Since la mousse is feminine singular, the pronoun is la, but before a vowel it becomes l’:

• la mousse → la
• la + essuyer → l’essuyer

So l’ here means it, referring to the foam.

In French, when an object pronoun goes with an infinitive, it usually comes directly before the infinitive.

So:

• essuyer la mousse = to wipe the foam
• l’essuyer = to wipe it

And after finir par, the pronoun stays with the infinitive:

• elle finit par l’essuyer

This is very normal in French.

Compare:

• Je veux le voir. = I want to see him/it.
• Nous allons la prendre. = We are going to take it.
• Elle finit par l’essuyer. = She ends up wiping it.

Because in French, each finite verb normally needs its own subject.

So French says:

• Marie rit..., puis elle finit...

and not normally:

• Marie rit..., puis finit...

In English, we can sometimes omit the repeated subject in certain styles, but French usually keeps it. Repeating elle sounds natural and correct.

Yes, the main verbs are in the present tense:

• rit = present
• finit = present

And voyant is a present participle.

In French, the present tense can describe:

• what is happening now
• a general truth
• a vivid narrative scene

So depending on context, this sentence could feel like:

• Marie is laughing ... then she ends up wiping it or
• a storytelling present: Marie laughs ... then she ends up wiping it

A rough pronunciation guide is:

Ma-rie ri an vwa-yan la mooss sur luh sol, pwee zell fin-ee par lay-swee-yay.

A few useful points:

• rit sounds like ri
• en voyant has a nasal sound in en
• puis elle is often pronounced smoothly together
• l’essuyer begins with a vowel sound, so the l’ links directly to it

If you want to be more precise:

• Marie → ma-ree
• rit → ree
• en voyant → roughly ahn vwa-yahn
• puis → roughly pwee
• l’essuyer → roughly lay-swee-yay

It can be a little tricky because the spelling changes in some forms.

Essuyer means to wipe.

Present tense:

• j’essuie
• tu essuies
• il/elle essuie
• nous essuyons
• vous essuyez
• ils/elles essuient

Notice the change:

• y becomes i in several forms
• but stays y with nous and vous

That is why the infinitive in the sentence is still essuyer, even though some conjugated forms look like essuie.

Yes, very often.

Finir par + infinitive often suggests that something happened:

• after some hesitation
• after some time
• after trying other things
• or simply as the final outcome

So this sentence can suggest a little mini-story:

• Marie sees the foam and laughs
• maybe she does not act immediately
• but eventually she wipes it up

That nuance is one reason finit par is so useful in French.