Avant de partir, Marie vérifie si le réservoir est plein et si les phares fonctionnent.

After avant, French normally uses de before an infinitive.

• avant de partir = before leaving
• not avant partir

This is a very common pattern:

• avant de manger = before eating
• avant de dormir = before sleeping

Here, partir stays in the infinitive because it is not the main conjugated verb of the sentence.

French often uses avant de + infinitive when the subject of both actions is the same.

In this sentence, Marie is the one who:

• checks things
• and leaves

So French prefers:

• Avant de partir, Marie vérifie...

rather than a longer structure like:

• Avant qu’elle parte, Marie vérifie...

That second version is possible in some contexts, but avant de + infinitive is simpler and more natural here because the subject is the same.

Because vérifie is the conjugated verb that matches the subject Marie.

The infinitive is vérifier = to check

In the present tense:

• je vérifie
• tu vérifies
• il/elle vérifie
• nous vérifions
• vous vérifiez
• ils/elles vérifient

Since the subject is Marie, we use the elle form:

• Marie vérifie

Here, si means whether or if in the sense of checking a fact.

So:

• Marie vérifie si le réservoir est plein = Marie checks whether the tank is full
• Marie vérifie si les phares fonctionnent = Marie checks whether the headlights are working

This is not the conditional if of sentences like If it rains, we stay home.
It introduces an indirect yes/no question.

French usually repeats si when two indirect yes/no clauses are joined.

So French prefers:

• Marie vérifie si le réservoir est plein et si les phares fonctionnent.

This makes it clear that she is checking two separate things:

1. whether the tank is full
2. whether the headlights work

In English, we may sometimes omit the second whether/if, but in French repeating it is very natural and often better style.

Here, plein is not directly describing the noun inside a noun phrase. It is being used with être as a predicate adjective.

Structure:

• le réservoir = the tank
• est = is
• plein = full

So literally:

• the tank is full

After être, adjectives normally come after the verb:

• La porte est ouverte.
• Le moteur est chaud.
• Le réservoir est plein.

So plein is in its normal place here.

Because plein agrees with le réservoir, which is masculine singular.

Agreement rules:

• masculine singular: plein
• feminine singular: pleine
• masculine plural: pleins
• feminine plural: pleines

Since réservoir is masculine singular, the correct form is:

• le réservoir est plein

If the noun were feminine, for example la bouteille, you would say:

• la bouteille est pleine

Because les phares is plural, so the verb must be in the plural form.

The infinitive is fonctionner = to function, to work

Present tense:

• il/elle fonctionne
• ils/elles fonctionnent

Since les phares means the headlights, plural, French uses:

• les phares fonctionnent

A useful pronunciation note: the -ent ending in the third person plural is usually not pronounced.
So fonctionne and fonctionnent sound the same in normal speech.

French uses articles more often than English does.

So French says:

• le réservoir
• les phares

Even where English might sometimes be less explicit, French generally wants the article with concrete nouns like these.

Here the speaker is referring to the car’s tank and headlights, so the definite articles are natural:

• the tank
• the headlights

Yes. Fonctionner is a very natural verb here. It means to function or more naturally in English, to work.

So:

• les phares fonctionnent = the headlights are working

French often uses fonctionner for machines, devices, systems, and parts of a vehicle.

Other verbs may be possible in different contexts, but they mean slightly different things:

• marcher = to work, to run, more informal
• s’allumer = to turn on, to light up

So if you want to say the headlights are in working order, fonctionner is an excellent choice.

Because Avant de partir is an introductory phrase placed at the beginning of the sentence.

It sets the time frame for the main action:

• Before leaving, Marie checks...

In French, as in English, a comma is commonly used after this kind of opening phrase to make the sentence easier to read.

So:

• Avant de partir, Marie vérifie...

is the normal punctuation.