Je vais écrire l’adresse sur l’enveloppe avant de partir.

Je vais écrire uses the near future: aller + infinitive. It often suggests something planned or about to happen soon, like English I’m going to write.

J’écrirai is the simple future, meaning I will write. That also works, but it can sound a little more neutral, less immediate, or slightly more formal depending on context.

So in this sentence, je vais écrire gives a natural sense of an intended next action.

No. In je vais écrire, vais is part of the grammar pattern aller + infinitive, which forms the near future.

So here it does not mean that the speaker is physically going somewhere in order to write. It simply means I am going to write.

Because after aller in the near future construction, the second verb stays in the infinitive.

So:

• je vais écrire
• tu vas écrire
• nous allons écrire

Only aller is conjugated. The main action verb, here écrire, stays unchanged.

Normally, la becomes l’ before a vowel sound. This is called elision.

So:

• la enveloppe becomes l’enveloppe
• la adresse would become l’adresse

However, there is an important spelling note here: in standard French, the noun is adresse, so the normal spelling is l'adresse with no accent on the a. If you see l’adresse, that is usually a typo or an autocorrect issue.

Because French uses sur for something written on the surface of an object.

So écrire l’adresse sur l’enveloppe means writing the address on the envelope.

If you said dans l’enveloppe, that would mean inside the envelope, which would not fit the idea of writing the address on the outside.

L’enveloppe is the definite article, so it refers to a specific envelope, one that is known from the context.

• l’enveloppe = the envelope
• une enveloppe = an envelope / one envelope

In this sentence, the speaker seems to have a particular envelope in mind, so l’enveloppe is natural.

After avant, French uses de before an infinitive.

So the pattern is:

avant de + infinitive

Examples:

• avant de partir
• avant de manger
• avant de dormir

So avant partir is not correct.

Because partir is an infinitive, and in this structure its subject is understood to be the same as the subject of the main clause.

Here, je is doing both actions:

• je vais écrire
• je vais partir

So French does not repeat je before partir.

Yes, but it is not the most natural choice here.

When the subject is the same in both parts of the sentence, French usually prefers:

avant de + infinitive

So avant de partir is the normal version here.

Avant que je parte is grammatically possible, but it is heavier and less usual in this situation. Also, after avant que, French uses the subjunctive, which is why it would be je parte, not je pars.

Partir usually means to leave / to depart, often with the idea of going away from a place.

Sortir usually means to go out, especially to go from inside to outside.

So:

• partir = leave, head off
• sortir = go out

In this sentence, partir is a natural general choice for before leaving. If the speaker specifically meant before going outside, sortir might be possible, but partir is broader and more neutral here.