Learn Hardcore French: Je mets le document dans ce dossier.

to put / place: Je mets le livre sur la table. (I put the book on the table.)
to put on (clothes): Je mets une veste. (I put on a jacket.)
to set (the table, an alarm): Je mets la table., Je mets mon réveil.

You could use other verbs like:

placer – more formal, like “to position / to place”:
Je place le document dans ce dossier. (Correct, but more formal or technical.)
ranger – “to put away / tidy away”:
Je range le document dans ce dossier. (Emphasizes putting it away in order.)

Mettre is the most neutral and common verb here.

Why is it dans and not another preposition like en, sur, or à?

Dans is used because the idea is inside a container:

dans = in / inside (physically or metaphorically in many cases)
Je mets le document dans ce dossier. → The document goes inside the folder/file.

Compare with:

sur = on / on top of:
Je mets le document sur le bureau. (on the desk, not inside)
en is often used with materials, means, duration, style:
un pull en laine (a wool sweater), en voiture (by car), en deux heures (in two hours)
à is very wide in use (to, at, in) but not used for “put X in a folder” in this sense.

So dans is the natural choice for putting something inside a folder/container.

Why do we say le document and not something like un document or leave the article out like in English sometimes?

French always needs an article (or a determiner) in front of singular countable nouns. So you cannot just say Je mets document dans ce dossier – that is wrong.

The choice is about which article:

le document = the document (a specific one both speaker and listener know about)
un document = a document (not a specific, known one)
ce document = this/that document (when you point or specify)

In the sentence Je mets le document dans ce dossier, we are talking about a specific, known document, so le is natural.

How do I know that document and dossier are masculine, and what changes if they were feminine?

You mostly have to memorize the gender of nouns. There is no completely reliable rule, though there are patterns. Here:

un document → masculine → le document
un dossier → masculine → ce dossier

Because they are masculine:

You use le (not la) → le document
You use ce (not cette) → ce dossier

If they were feminine, you would see:

la lettre, cette lettre (letter)
la chemise, cette chemise (paper folder)

So the article (le / la) and the demonstrative (ce / cette) change with the gender.

What exactly does dossier mean here? Is it a folder, a file, or something else? How is it different from fichier?

Dossier usually means:

A folder (physical or digital) containing documents, or
A file in the sense of a case file (e.g., a medical file, a legal file).

Typical uses:

un dossier papier → a paper folder or file
un dossier médical → a medical file
On a computer: un dossier is a folder that can contain files.

Fichier is different:

un fichier = a file in the computer sense (a single data file: .doc, .pdf, etc.)

So:

Je mets le document dans ce dossier. → I put the document into this folder / case file.
Je sauvegarde le fichier sur mon ordinateur. → I save the file on my computer.

Why is it ce dossier and not cet dossier or cette dossier? How do ce, cet, cette, and ces work?

Ce / cet / cette / ces are all forms of this / that / these / those.

They agree in gender and number:

ce – masculine singular before a consonant: ce dossier, ce document
cet – masculine singular before a vowel or mute h: cet homme, cet ordinateur
cette – feminine singular: cette lettre, cette chemise
ces – plural (masc. or fem.): ces dossiers, ces lettres

Since dossier is masculine and starts with a consonant sound, you must use ce dossier.

Could I say Je le mets dans ce dossier or J’y mets le document? What changes when I use pronouns?

Yes, both are correct but they emphasize different things.

Je le mets dans ce dossier.
- le replaces le document (direct object pronoun).
- Meaning: I put it in this folder.
- Structure: subject (je) + pronoun (le) + verb (mets) + rest.
J’y mets le document.
- y replaces a previously mentioned place like dans ce dossier.
- Meaning: I put the document in it / there.
- Structure: J’y mets le document. (remember je
  - y → J’y for pronunciation)

You can also replace both when the context is clear:

Je le mets dedans. → I put it inside (there).

Pronoun order with mettre in the present is important: they go before the verb.

Is the word order fixed? Can I say Je mets dans ce dossier le document?

The neutral, most natural order is:

Je mets le document dans ce dossier.
subject – verb – direct object – location

You can say:

Je mets dans ce dossier le document.

But this sounds a bit marked or emphatic, like you’re insisting on in this folder as new or important information. It is grammatically correct but less usual in everyday speech.

For standard usage, keep the original order.

How is Je mets le document dans ce dossier understood in terms of time? Is it “I put”, “I am putting”, or “I will put”?

In French, the present tense (je mets) can cover several English tenses, depending on context:

I put (habitual):
Chaque fois, je mets le document dans ce dossier.
I am putting (right now):
Là, je mets le document dans ce dossier.
I’ll put / I’m going to put (near future, in some contexts):
Attends, je mets le document dans ce dossier et j’arrive.

Without extra context, Je mets le document dans ce dossier is usually understood as a present action or a general habit.

How do you pronounce the sentence, especially mets and the liaison?

One natural pronunciation (in IPA) is:

Je mets le document dans ce dossier.
→ [ʒə mɛ lə dɔ.ky.mɑ̃ dɑ̃ sə dɔ.sje]

Key points:

mets → [mɛ]; the -ts is silent.
There is usually a liaison between dans and ce: [dɑ̃‿sə] (you hear a [z] sound? Actually [dɑ̃s.sə] → kind of [s] continuing; in many accents it’s smooth).
document → [dɔ.kɥə.mɑ̃] or [dɔ.kʏ.mɑ̃] depending on accent (you’ll often hear [dɔ.kymɑ̃]).
Final -r in dossier is silent: [dɔ.sje].

Can I drop the subject je like in Spanish or Italian and just say Mets le document dans ce dossier?

No, not in a normal statement.

In French, the subject pronoun (je, tu, il, etc.) is almost always required because the verb ending alone usually does not clearly show the subject.

Je mets le document dans ce dossier. → correct
Mets le document dans ce dossier. → this is imperative (“Put the document in this folder”), not the same meaning.

So for a simple statement, keep je.

How would this sentence change for you or they instead of I?

Change only the subject pronoun and the verb ending:

Tu mets le document dans ce dossier. → You (singular, informal) put the document in this folder.
Il / Elle met le document dans ce dossier. → He / She puts…
Nous mettons le document dans ce dossier. → We put…
Vous mettez le document dans ce dossier. → You (formal or plural) put…
Ils / Elles mettent le document dans ce dossier. → They put…

Notice how irregular the endings are: mets, mets, met, mettons, mettez, mettent.

Is dans ce dossier okay for both a physical folder and a computer folder?

Yes.

For a physical folder:
Je mets le document dans ce dossier. → I physically put the paper document inside this folder.
For a computer folder:
Je mets le document dans ce dossier. → I move/save the digital document into this folder (on the computer).

Context (physical vs digital) will usually be clear from the situation or previous sentences.