Je mets la copie dans l’enveloppe avant de partir.

Because the verb is mettre (to put), and with je in the present tense, the correct form is mets.

Present tense of mettre:

• je mets
• tu mets
• il / elle / on met
• nous mettons
• vous mettez
• ils / elles mettent

So je mets la copie... means I put / I am putting the paper...

Also, the -s in mets is usually not pronounced.

French usually needs an article where English often does not.

So French says:

• la copie = the copy / the paper
• l’enveloppe = the envelope

In English, you might sometimes say I put the paper in an envelope or even leave out an article in other contexts, but French is much stricter about using articles with nouns.

Here, la copie refers to a specific paper or copy, not just the general idea of copy.

Because enveloppe begins with a vowel sound, la contracts to l’.

So:

• la enveloppe → l’enveloppe

This happens with:

• le
  • vowel/h mute → l’
• la
  • vowel/h mute → l’

Examples:

• l’école
• l’ami
• l’heure
• l’enveloppe

Here, dans means in / inside and is the natural choice for something being placed physically inside something else.

So:

• dans l’enveloppe = in the envelope

French often uses dans for literal physical location or movement into something:

• dans la boîte = in the box
• dans le sac = in the bag

En can also mean in, but it is used differently and would not normally be the best choice here.

Because after avant, French normally uses de before an infinitive.

So:

• avant de partir = before leaving / before I leave

This is a very common structure:

• avant de manger = before eating
• avant de dormir = before sleeping
• avant de répondre = before answering

So de is required here.

French often uses avant de + infinitive when the subject is the same as in the main clause.

Here, the subject is je in both actions:

• Je mets...
• je pars

So French prefers:

• Je mets la copie dans l’enveloppe avant de partir.

If the subject changes, French usually uses:

• avant que + subjunctive

For example:

• Je mets la copie dans l’enveloppe avant qu’il parte. = I put the paper in the envelope before he leaves.

So:

• same subject → avant de + infinitive
• different subject → avant que + subjunctive

It can mean both, depending on context.

French present tense often covers:

• I put
• I am putting
• sometimes even a near-future sense, depending on the situation

So Je mets la copie dans l’enveloppe could mean:

• I put the paper in the envelope
• I’m putting the paper in the envelope

French does not always need a separate form like English am putting.

Copie can mean several things depending on context, such as:

• a copy
• a paper
• a test paper
• a written assignment
• sometimes a duplicate

In a sentence like this, it often means a physical paper or document being placed into an envelope.

English speakers should be careful not to assume it always means only copy in the narrow English sense.

Because partir is in the infinitive form, not a conjugated verb.

In English, we often say:

• before leaving or
• before I leave

French uses:

• avant de partir

The subject je is understood from the main clause because the subject is the same.

So French does not need to repeat je here.

Yes. That is perfectly correct.

Both are natural:

• Je mets la copie dans l’enveloppe avant de partir.
• Avant de partir, je mets la copie dans l’enveloppe.

The meaning is basically the same. The difference is mainly emphasis and rhythm:

• starting with Avant de partir highlights the time frame first
• starting with Je mets... highlights the action first

Mets is pronounced roughly like may.

A few useful points:

• the -s is silent
• mets, met, and mais can sound very similar or identical in many accents
• je mets sounds roughly like zhuh may

So although mets is spelled with -ts, you do not pronounce those final consonants in normal speech.

Because partir means to leave / to go away / to depart, while quitter usually takes a direct object and means to leave something or someone.

So:

• partir = to leave, to go away
• quitter la maison = to leave the house
• quitter Marie = to leave Marie

In this sentence, the idea is simply before leaving, not before leaving something specific, so partir is the right choice.

Yes. It follows a very common pattern:

• subject: Je
• verb: mets
• direct object: la copie
• place expression: dans l’enveloppe
• time/action phrase: avant de partir

This is very natural French word order. English speakers can often understand it as:

I + put + the paper + in the envelope + before leaving

So the sentence is structurally straightforward and idiomatic.