Quand la tarte est presque prête, Marie la garnit de fines tranches de pomme.

Quand introduces a time clause. It tells you at what point the main action happens.

Here, the structure is:

Quand + clause, main clause

So:

Quand la tarte est presque prête = the moment/once the tart is almost ready
Marie la garnit... = Marie does the next action

In a sentence like this, quand can often feel like when or once, depending on context.

Because prête agrees with la tarte, which is feminine singular.

The adjective is prêt / prête = ready.

So you get:

• le gâteau est prêt
• la tarte est prête

French adjectives usually agree in gender and number with the noun they describe.

No. Here, est prête is simply être + adjective.

It means the tart is ready / is almost ready.
It is not a passive like is prepared or is made.

So:

• La tarte est prête = the tart is ready
• not a passive construction

La is a direct object pronoun referring back to la tarte.

Instead of repeating la tarte, French replaces it with la:

• Marie garnit la tarte
• Marie la garnit

In French, object pronouns usually come before the conjugated verb, which is different from English.

Garnir often means to garnish, fill, top, line, or cover, depending on context.

In this sentence, it means something like:

• to top the tart
• to cover it with
• to arrange on it

So it is a very natural cooking verb here.

Because garnir commonly uses the pattern:

garnir quelque chose de quelque chose

So French naturally says:

• garnir une tarte de pommes
• garnir une assiette de persil

Using avec is not the usual choice with this verb. A native speaker expects de after garnir.

The first de belongs to the construction garnir de.

So the sentence is built like this:

• Marie la garnit de ...
• Marie garnishes it with ...

Then fines tranches is the thing being added.

Also, French often uses bare de in this kind of structure before an indefinite quantity or plural idea, especially after verbs like garnir, couvrir, remplir, and similar verbs.

Because French adjective position is not always the same as in English.

Here, fine means thin or delicate, and de fines tranches is a very natural culinary expression.

So:

• de fines tranches de pomme sounds idiomatic
• des tranches fines de pomme is possible in some contexts, but it sounds less natural here and more marked

In recipes and food descriptions, de fines tranches is very common.

Here, pomme is used in a very common French ingredient/type pattern:

• tranches de pomme
• slices of apple

The singular often names the substance or ingredient in a general way, rather than counting separate apples.

It is similar to how French can use singular nouns after de to indicate type, material, or ingredient.

So de pomme here feels like apple slices as a topping, not slices from several individual apples.

Both est and garnit are in the present indicative.

French often uses the present tense for:

• recipes
• instructions
• habitual actions
• narration of a sequence

So this sentence sounds like a recipe step or a descriptive instruction:

When the tart is almost ready, Marie adds the apple slices.

English can do something similar, but French uses this present-tense style very naturally.

Because the sentence begins with a subordinate time clause:

Quand la tarte est presque prête

and then moves to the main clause:

Marie la garnit de fines tranches de pomme.

The comma helps separate the two parts and makes the sentence easier to read. It is very normal here.

Yes. Both are grammatically correct.

• Marie garnit la tarte de fines tranches de pomme
• Marie la garnit de fines tranches de pomme

Using la avoids repeating la tarte, so it sounds smoother and more natural once the tart has already been mentioned.