La caissière nous donne un ticket de caisse, mais mon fils veut déjà sortir.

La means the, while une means a / an.

French often uses the when talking about a specific person in the situation, even if English might say the cashier or sometimes just the cashier at the store. Here, la caissière refers to the cashier involved in this scene.

Also, caissière is a feminine noun, so it takes la.

• le caissier = the male cashier
• la caissière = the female cashier

French nouns have grammatical gender. Caissière is the feminine form of caissier.

This matches the fact that the cashier being talked about is female. In many job titles, French has different masculine and feminine forms:

• un boulanger / une boulangère
• un infirmier / une infirmière
• un caissier / une caissière

So la caissière means the person is both:

• grammatically feminine
• actually female in this context

Because nous here is an object pronoun, and in French object pronouns normally go before the conjugated verb.

So:

• La caissière nous donne... = The cashier gives us...

Compare:

• Elle nous parle. = She talks to us.
• Il me voit. = He sees me.
• Tu lui écris. = You write to him/her.

This is different from English, where object pronouns usually come after the verb:

• gives us
• sees me

French usually does:

• nous donne
• me voit

Here it means us.

French nous can mean either:

• we as a subject
• us / to us as an object

You can tell from the sentence structure:

• Nous donnons... = We give...
• La caissière nous donne... = The cashier gives us...

In your sentence, la caissière is the subject, so nous cannot mean we. It is the object.

Because the subject is la caissière, which is third person singular: she.

The verb is donner = to give. In the present tense:

• je donne
• tu donnes
• il/elle/on donne
• nous donnons
• vous donnez
• ils/elles donnent

Since la caissière = she, the correct form is donne.

Because ticket is a masculine singular noun, so the indefinite article is un.

• un ticket
• not une ticket

French articles must match the gender and number of the noun:

• un livre
• une table
• des tickets

So un ticket de caisse means a receipt / a cash-register receipt.

In French, de + noun is often used to describe what type of thing something is.

So ticket de caisse literally means something like:

• cash-register ticket
• checkout receipt

This is a very common French pattern:

• une tasse de thé = a cup of tea
• une salle de bain = a bathroom
• un ticket de caisse = a store receipt / receipt from the register

Using de la caisse would sound more specific, like the receipt of the register or from the register, which is not the normal fixed expression here.

Yes, it is very common in everyday French, especially in shops.

A few useful points:

• un ticket de caisse = the receipt you get in a store
• un reçu = a receipt more generally, often in formal or financial contexts

So in a supermarket or shop, ticket de caisse is very natural.

Because fils is a masculine noun.

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

So:

• mon fils = my son
• ma fille = my daughter

It does not matter whether the speaker is male or female. A mother still says:

• mon fils
• ma fille

because the possessive adjective matches fils or fille, not the speaker.

Because after a conjugated verb like vouloir (to want), French usually puts the second verb directly in the infinitive.

So:

• veut sortir = wants to go out / wants to leave

French does not add a separate word equivalent to English to here.

More examples:

• Je veux manger. = I want to eat.
• Elle aime lire. = She likes to read.
• Nous allons partir. = We are going to leave.

The infinitive itself already covers the idea of to + verb.

Déjà usually means already.

In this sentence, it suggests that the son wants to leave so soon or earlier than expected.

French adverbs like déjà often go after the conjugated verb and before the infinitive or the rest of the sentence:

• veut déjà sortir

Compare:

• Il a déjà mangé. = He has already eaten.
• Elle veut déjà partir. = She already wants to leave.

So the placement is normal French word order.

Here sortir means something like to go out or to leave.

Depending on context, sortir can mean:

• to go out
• to go outside
• to leave
• to come out

In this sentence, because they are at a checkout, mon fils veut déjà sortir most naturally means he wants to leave the store or go outside already.

French often uses the present tense to describe what is happening in a scene, story, or immediate situation.

So:

• La caissière nous donne...
• mon fils veut...

This is just normal present tense narration.

English does something similar:

• The cashier gives us a receipt, but my son already wants to leave.

Depending on the wider context, English might also choose is giving or wants to, but French simple present is very common here.

Mais means but.

It connects two ideas that contrast slightly:

• the cashier is still giving the receipt
• meanwhile, the son already wants to leave

So mais shows a contrast between what is happening and the son’s impatience.

Yes, that is possible, but it changes the feel slightly.

• un ticket de caisse = a receipt, introducing it as a thing being given
• le ticket de caisse = the receipt, referring to that specific receipt more definitely

In many situations, both are possible. Un often sounds a bit more natural when simply stating what the cashier gives you as part of the transaction.