Avant de partir, ma fiancée met la bague dans sa poche et vérifie une dernière fois la réservation.

Because after avant, French normally uses de + infinitive when the subject stays the same.

So:

• avant de partir = before leaving

Here, the person who is leaving is the same person doing the other actions, so French uses avant de + infinitive.

If the subject changes, French usually switches to avant que + subjunctive:

• Avant qu’elle parte, je téléphone.
  Before she leaves, I call.

The difference is grammatical gender:

• fiancé = an engaged man / fiancé
• fiancée = an engaged woman / fiancée

Since the sentence talks about a woman, French uses ma fiancée.

The extra -e marks the feminine form. In writing, that difference matters even though the pronunciation is essentially the same.

Because the verb is conjugated for elle:

• je mets
• tu mets
• il / elle / on met

So with ma fiancée, the correct form is met.

This comes from the irregular verb mettre.

Because the same subject is doing both actions:

• ma fiancée met...
• et vérifie...

French, like English, does not need to repeat the subject when one person does two verbs joined by et.

So this works just like:

• She puts on the ring and checks the reservation.

You could repeat the subject, but it would usually sound unnecessary here.

In French, possessive adjectives agree with the thing possessed, not with the owner.

Since poche is a feminine singular noun, French uses:

• sa poche

That form can mean:

• his pocket
• her pocket
• its pocket

The context tells you whose pocket it is. Here, because the subject is ma fiancée, we naturally understand her pocket.

French often uses the definite article when talking about a specific thing already understood from the situation.

So:

• la bague = the ring
• la réservation = the reservation

The sentence suggests there is one particular ring and one particular reservation that both speaker and listener can identify from context.

French uses definite articles a bit more broadly than English does, so learners often notice this.

Une dernière fois means one last time.

It is a very common expression for doing something once more before finishing.

So:

• vérifie une dernière fois = checks one last time

By contrast, la dernière fois usually means the last time in the sense of a previous occasion:

• La dernière fois, il était en retard.
  Last time, he was late.

So in this sentence, une dernière fois is the natural choice.

Because une dernière fois is modifying the action vérifie.

French often places this kind of time/frequency expression right after the verb:

• vérifie une dernière fois la réservation

That word order is very natural.

You may also hear:

• vérifie la réservation une dernière fois

That is also possible, but the original version puts the focus a little more directly on the idea of checking one final time.

Yes. The main verbs are in the present tense:

• met
• vérifie

French present tense can be used in several ways:

• for what is happening now
• for habits or routines
• for vivid storytelling

So without more context, the sentence could describe:

• what she is doing now
• what she usually does before leaving
• a narrative scene

The phrase avant de partir simply gives the time relationship: these actions happen before leaving.

Here are a few useful approximations:

• fiancée ≈ fee-ahn-SAY
  The -an- sound is nasal in real French.
• met ≈ meh
• bague ≈ bahg
• vérifie ≈ vay-ree-FEE
• réservation ≈ ray-zair-va-SYON

A few pronunciation notes:

• Final consonants are often silent in French.
• The accents in fiancée, vérifie, and réservation help indicate the vowel sound.
• The ending -tion in réservation is pronounced syon, not like English tee-on.