La copie du justificatif est acceptée aujourd’hui, mais demain il faudra montrer l’original.

Because du is the normal contraction of de + le in French.

Here, copie de means copy of, so:

• de + le justificatif → du justificatif

This kind of contraction is required in standard French.

Related patterns:

• de + le → du
• de + les → des
• de + la stays de la
• de + l’ stays de l’

Examples:

• la copie du document
• la copie de la facture
• la copie de l’attestation
• la copie des pièces

In administrative French, un justificatif usually means a supporting document or proof.

It is often shorthand for a fuller expression such as:

• justificatif de domicile = proof of address
• justificatif d’identité = proof of identity
• justificatif de revenus = proof of income

So in this sentence, le justificatif means the supporting document being discussed.

Because it agrees with la copie, which is feminine singular.

The structure is passive:

• La copie = the subject
• est acceptée = is accepted

In French, when a past participle is used with être, it agrees with the subject.

So:

• la copie est acceptée → feminine singular
• le document est accepté → masculine singular
• les copies sont acceptées → feminine plural
• les documents sont acceptés → masculine plural

The extra -e is there because copie is feminine.

Yes. It is the passive voice.

• La copie du justificatif est acceptée = The copy of the document is accepted

This focuses on the thing being accepted, not on who accepts it.

A more active version would be something like:

• On accepte la copie du justificatif aujourd’hui

Both are possible, but the passive is very common in formal notices, administrative language, and official instructions.

French uses articles much more often than English does.

So where English might say something like:

• copy accepted today, original required tomorrow

French normally needs articles:

• la copie
• l’original

Here the definite article refers to the relevant copy and the relevant original in the situation being discussed.

This is especially common in notices and administrative language, where French often talks about things in a general-but-specific way:

• La carte d’identité est demandée
• Le passeport est nécessaire
• L’original doit être présenté

This is the future tense of falloir, which means to be necessary or to have to.

Falloir is an impersonal verb, so it is normally used only with il:

• il faut = it is necessary / you have to
• il fallait = it was necessary
• il faudra = it will be necessary / you will have to

In this sentence:

• demain il faudra montrer l’original

literally means:

• tomorrow, it will be necessary to show the original

In natural English, that usually becomes:

• tomorrow you will have to show the original

The il does not refer to a person or thing. It is just the fixed impersonal subject used with falloir.

Because after falloir, the next verb normally stays in the infinitive.

So:

• il faut montrer
• il faudra montrer
• il fallait montrer

This works much like English to have to plus a base verb:

• you have to show
• you will have to show

Other examples:

• Il faut attendre
• Il faudra signer
• Il faut apporter une pièce d’identité

So montrer is not conjugated because faudra is already the conjugated verb.

Because demain is an adverb of time, not a noun here.

Just like aujourd’hui, it directly means a time reference:

• aujourd’hui = today
• demain = tomorrow

French does not normally use an article before these adverbs.

So:

• aujourd’hui la copie est acceptée
• demain il faudra montrer l’original

That is normal French word usage.

If demain were used as a noun in a more literary or special context, things could be different, but not here.

Because original starts with a vowel sound, and le or la becomes l’ before a vowel or mute h.

So:

• le original → not correct
• l’original → correct

This is called elision.

Other examples:

• l’identité
• l’adresse
• l’attestation
• l’hôtel

Here, l’original means the original document.

Yes. Just like in English, an adjective can sometimes be used as a noun when the meaning is understood.

So:

• l’original = the original one / the original document

In this sentence, it clearly means the original version of the supporting document.

French does this quite often when the noun is obvious from context.

For example:

• une copie / l’original
• le premier / le second
• la suivante

So montrer l’original is completely natural.

Because French often places time expressions after the verb or after the main verbal phrase.

So:

• La copie du justificatif est acceptée aujourd’hui

is a very normal word order.

You could also move aujourd’hui for emphasis in some contexts:

• Aujourd’hui, la copie du justificatif est acceptée

Both are correct, but the version in your sentence sounds very natural and neutral.

Yes, but it would change the feel slightly.

• La copie du justificatif sounds like a general rule or official statement about the copy of that document type.
• Une copie du justificatif would sound more like a copy of the document is acceptable.

In administrative French, the definite article is very common in instructions and notices, so la copie sounds especially natural here.

Both can be grammatical, but la copie fits the formal notice style well.