Je garde une copie du contrat, mais l’original reste dans le dossier bleu.

Because du is the mandatory contraction of de + le.

• une copie de le contrat is not correct French
• de + le = du
• so une copie du contrat means a copy of the contract

This happens regularly in French:

• le livre du professeur = the teacher’s book / the book of the teacher
• la porte du bureau = the office door

Here, contrat is masculine singular, so de + le becomes du.

Because le becomes l’ before a vowel sound.

Since original begins with o, French uses elision:

• le original → l’original

This is very common:

• l’homme
• l’école
• l’idée

So l’original simply means the original, with the normal French article change before a vowel.

Here, original is a noun.

In this sentence:

• une copie = a copy
• l’original = the original document

So original is not describing another noun; it is standing on its own and means the original version or the original document.

English does this too:

• I kept the copy, but the original stayed in the file.

The articles are different because the speaker is presenting the two items differently.

• une copie = a copy → an indefinite item, one copy
• l’original = the original → a specific, identifiable document

French uses:

• un / une for a / an
• le / la / l’ for the

So the sentence contrasts:

• a copy with
• the original

That makes perfect sense, because there may be many copies, but only one original.

Je garde means something like I keep, I’m keeping, or I retain here.

The verb garder often means:

• to keep
• to hold on to
• to retain
• to leave something in your possession

In this sentence, Je garde une copie du contrat means the speaker keeps a copy for themselves.

It does not mean to look after here, although garder can also mean that in other contexts, as in garder un enfant = to look after a child.

It can mean either, depending on context.

French present tense often covers both:

• English simple present: I keep
• English present continuous: I am keeping

So Je garde une copie du contrat could mean:

• I keep a copy of the contract or
• I’m keeping a copy of the contract

French usually does not need a separate am keeping form the way English does.

Because rester adds the idea of remaining or staying.

• est dans le dossier bleu = is in the blue file
• reste dans le dossier bleu = remains / stays in the blue file

So reste is a little more precise here. It suggests that the original is left there and continues to be there, while the speaker keeps only a copy.

That makes the contrast stronger:

• I keep a copy
• but the original stays in the blue file

Because in French, most color adjectives come after the noun.

So:

• le dossier bleu = the blue file

This is normal French word order. Other examples:

• une voiture rouge = a red car
• un sac noir = a black bag

French adjectives do not always come after the noun, but color adjectives usually do.

Because bleu has to agree with dossier, and dossier is masculine singular.

So:

• un dossier bleu = masculine singular
• une chemise bleue = feminine singular
• des dossiers bleus = masculine plural
• des chemises bleues = feminine plural

In your sentence, dossier is masculine singular, so bleu stays in its basic masculine singular form.

Here, dossier most likely means file or folder.

Depending on context, dossier can mean:

• a file
• a folder
• a case file
• a set of documents

In le dossier bleu, the most natural meaning is probably the blue file/folder where the original document is kept.

Because dans means in or inside.

So:

• dans le dossier bleu = in the blue file

French uses dans when something is physically inside something else, or conceptually contained in it.

That is the natural preposition here, because the original document is located inside the file/folder.

It is connected to copie.

The structure is:

• Je garde = I keep
• une copie du contrat = a copy of the contract

So du contrat tells you what kind of copy it is.

It does not mean I keep from the contract. Instead, it means:

• a copy of the contract

You can think of une copie du contrat as one noun phrase.