Quand j’aurai fini mes fiches, je les rangerai dans mon casier au lieu de les laisser sur la table.

J’aurai fini is the future perfect in English terms, called the futur antérieur in French.

It is made with:

• the future tense of avoir: j’aurai
• plus the past participle: fini

So:

• j’aurai fini = I will have finished

French uses this tense to show that one future action will be completed before another future action.

This is a very common difference between English and French.

In English, after when, we usually use the present tense for future meaning:

• When I finish, I’ll put them away.

In French, after quand, you normally use a future form if the meaning is future:

• Quand j’aurai fini..., je les rangerai...

So even though English says when I finish, French often says the equivalent of when I will have finished.

That is completely normal in French.

Both can be possible in some contexts, but they do not mean exactly the same thing.

• Quand je finirai = when I finish / when I will finish
• Quand j’aurai fini = when I have finished / once I’m done

In your sentence, j’aurai fini is used because the speaker wants to emphasize that the finishing will be completed first, and only then will they put the cards away.

So the sequence is:

1. finish the cards
2. put them away

The futur antérieur is especially good for showing this completed-before-another-future-action idea.

Both instances of les refer to mes fiches.

• je les rangerai = I will put them away
• de les laisser sur la table = instead of leaving them on the table

French repeats the object pronoun because it belongs to each verb separately:

• ranger → put them away
• laisser → leave them

So English also does something similar:

• I’ll put them away instead of leaving them on the table

The them is understood with both actions, and French shows that explicitly.

Because les here is an object pronoun, not the definite article the.

French contracts de + les into des only when les is an article:

• de les livres becomes des livres

But in your sentence, les means them, not the:

• au lieu de les laisser = instead of leaving them

Since it is a pronoun, there is no contraction.

So:

• de les = correct here
• des = not correct here

Because les is the direct object of the infinitive laisser.

In French, object pronouns usually go before the verb they belong to.

So:

• les laisser = to leave them
• les ranger = to put them away

In the full sentence:

• je les rangerai → les goes before the conjugated verb rangerai
• au lieu de les laisser → les goes before the infinitive laisser

This is very normal French word order.

Au lieu de means instead of.

It is followed by:

• a noun, or
• an infinitive verb

In your sentence, it is followed by an infinitive:

• au lieu de les laisser sur la table
• instead of leaving them on the table

More examples:

• au lieu de travailler = instead of working
• au lieu du café = instead of the coffee

So in your sentence, au lieu de introduces the alternative action that the speaker is not going to do.

Because with avoir, the past participle usually does not agree with the direct object if that object comes after the verb.

Here:

• j’aurai fini mes fiches

The direct object mes fiches comes after fini, so the past participle stays in the default form:

• fini

So:

• j’aurai fini mes fiches = correct
• j’aurai finies mes fiches = incorrect

This is a more advanced grammar point, but it explains why fini does not change here.

In this sentence:

• fiches usually means cards, index cards, revision cards, study notes, or file cards, depending on context
• casier usually means locker, cubby, pigeonhole, or storage compartment

So mes fiches could be study cards, note cards, or some sort of organized sheets/cards.

And mon casier is the place where the speaker stores them.

The exact translation depends on context, but the French words themselves are very common.

Because the whole situation is in the future.

The sentence describes two future actions:

1. j’aurai fini = I will have finished
2. je les rangerai = I will put them away

French marks both as future because both actions happen later than now.

So the structure is:

• When I have finished X, I will do Y

In French:

• Quand j’aurai fini X, je ferai Y

That is why rangerai is in the simple future, called the futur simple.

Yes, very natural.

• laisser = to leave
• sur la table = on the table

So:

• les laisser sur la table = to leave them on the table

This is a completely normal everyday expression in French. It suggests that the speaker does not want the cards to stay lying there, and prefers to put them away neatly instead.