Au restaurant, Paul laisse toujours un petit pourboire quand le service est rapide et qu’on apporte vite une carafe d’eau.

Au restaurant usually means at the restaurant in a general, everyday sense: it sets the scene or context.

• Au restaurant = at/in a restaurant, as an activity or setting
• Dans le restaurant = physically inside the restaurant building

So here, Au restaurant sounds more natural because the sentence is describing what Paul does in that context, not emphasizing his physical location.

Laisse is the 3rd person singular present tense of laisser.

Here the subject is Paul, which is third person singular, so:

• je laisse = I leave
• tu laisses = you leave
• il / elle / on laisse
• Paul laisse

In this sentence, laisser un pourboire means to leave a tip.

In French, short adverbs like toujours, souvent, déjà, bien, etc. often come after the conjugated verb.

So:

• Paul laisse toujours un petit pourboire

This is the normal word order. English often puts always before the main verb (Paul always leaves...), but French usually puts toujours after it.

Yes, petit literally means small, but here it often has a more natural English sense like:

• a small tip
• a little tip

It does not necessarily mean tiny in a dramatic way. It just softens the expression and sounds natural. French often uses adjectives like this in ordinary speech.

Also, pourboire is a masculine noun, which is why it is:

• un petit pourboire

and not une petite pourboire.

The French present tense is often used to express habitual actions, just like the English present simple.

So here:

• Paul laisse toujours...
• le service est rapide
• on apporte...

all describe things that happen generally or typically, not just one specific moment.

So the sentence means Paul has this regular habit when those conditions are met.

Use quand when you mean when in the sense of at the time that / whenever.

• quand le service est rapide = when the service is fast

Use si for if, meaning a condition or uncertainty.

In this sentence, the idea is whenever the service is fast and water is brought quickly, Paul leaves a tip. So quand is the right choice.

Because rapide here describes the noun service, not the action of serving.

• le service = the service
• est rapide = is fast

So rapide is an adjective agreeing with service.

If you used an adverb, it would describe how someone does something, not what the service is like.

Compare:

• Le service est rapide = The service is fast.
• On sert rapidement = People serve quickly.

Both are possible in French, but they mean slightly different things.

This is a very common structure in French.

The sentence has quand introducing a subordinate clause:

• quand le service est rapide

Then a second idea is added, still under the same quand:

• et qu’on apporte vite une carafe d’eau

French commonly repeats the linking word with que:

• quand X et que Y

This is especially natural when the subject changes:

• first clause: le service
• second clause: on

So et qu’on is the normal, idiomatic form here.

Also, que + on becomes qu’on because of elision.

Here on means something like:

• people
• they
• someone
• the staff, depending on context

It is an indefinite subject pronoun. In this sentence, it refers generally to whoever is serving in the restaurant.

So:

• qu’on apporte vite une carafe d’eau

means when a carafe of water is brought quickly or when they bring a carafe of water quickly.

Even when on refers to people in a general sense, it is grammatically treated as third person singular.

So you say:

• on apporte
• not on apportent

This is one of the important things to remember about on: its meaning may feel plural in English, but its verb form is singular in French.

Both are possible, but vite is more common and more natural in everyday spoken and written French.

• vite = quickly
• rapidement = quickly, in a slightly more formal or more explicitly adverbial way

So:

• on apporte vite une carafe d’eau

sounds very natural.

French often prefers vite in ordinary speech.

Because the sentence is talking about a carafe of water, not just some water.

• de l’eau = some water / water
• une carafe d’eau = a carafe of water

A carafe is the container, so the phrase is more specific. In many restaurant contexts, that detail matters.

Also, d’eau is simply de + eau, with elision:

• de eau becomes d’eau

The comma separates the introductory phrase from the main clause.

• Au restaurant, = in the restaurant context / when at a restaurant
• Paul laisse toujours... = main statement

This kind of comma is very common when a sentence begins with a time/place/context expression. It helps readability, though in some short sentences French punctuation can be a little flexible.

A natural pronunciation would link the words smoothly, with the apostrophes showing dropped vowels:

• qu’on sounds roughly like kon
• apporte sounds like a-port
• vite sounds like veet
• une sounds like ewn
• carafe d’eau sounds roughly like ka-raf do

A very rough pronunciation of the whole chunk is:

kon a-port veet ewn ka-raf do

A few things to notice:

• qu’on is pronounced as one unit
• d’eau also flows together as one unit
• French rhythm is smoother and less stress-heavy than English

In standard, natural French, et qu’on is much better.

Without que, the sentence sounds awkward because the second clause is still meant to depend on quand. French normally marks that dependency clearly:

• quand le service est rapide et qu’on apporte vite une carafe d’eau

That structure shows both things are part of the same when/whenever condition.

So for a learner, the safest pattern to remember is:

• quand + clause 1 + et que + clause 2