Sur l’enveloppe, l’expéditeur doit aussi écrire sa propre adresse.

Sur l’enveloppe means on the envelope. It is placed at the beginning to set the scene first: the sentence tells you where this writing should appear.

French often moves a place phrase to the front for emphasis or clarity. A more neutral order would be:

L’expéditeur doit aussi écrire sa propre adresse sur l’enveloppe.

Both are correct. Starting with Sur l’enveloppe just highlights the location.

The comma marks a pause after the introductory phrase. In English, we often do something similar with a fronted phrase like On the envelope, ...

It helps show that Sur l’enveloppe is setting the context for the rest of the sentence. In short sentences, punctuation can vary a little, but the comma is very natural here.

They show elision: a final vowel drops before a word beginning with a vowel sound.

• la enveloppe becomes l’enveloppe
• le expéditeur becomes l’expéditeur

French does this very often to avoid awkward vowel clashes.

French usually needs an article where English may not.

Here, l’expéditeur means the sender. In context, it refers to the person sending the letter. Even in general instructions, French often uses the definite article for roles like this:

• le client = the customer
• le passager = the passenger
• l’expéditeur = the sender

So this sounds natural in French, even if English might sometimes say simply sender in a label or instruction.

Expéditeur means sender or the person who sends something.

In the context of a letter or parcel, it is the person sending it, not the person receiving it. So it is basically the opposite of destinataire, which means recipient/addressee.

Because doit is a conjugated form of devoir and it is followed by an infinitive.

• devoir = to have to / must
• doit = he/she must, or in this case the sender must
• écrire = to write

So doit écrire works like must write in English:

• Il doit écrire = He must write

After modal-type verbs like devoir, the next verb normally stays in the infinitive.

In French, short adverbs like aussi often go after the conjugated verb.

So:

l’expéditeur doit aussi écrire...

means:

the sender must also write...

That is the most natural placement here. It shows that this is an additional thing the sender has to do.

You may see different placements in other sentences, but doit aussi écrire is standard and very natural.

This is a very common question.

There are really two things happening:

1. propre adds emphasis

   • son adresse = his/her address
   • sa propre adresse = his/her own address

2. sa is used because adresse is feminine, and here the possessive is directly followed by propre, which begins with a consonant sound.

Normally, before a feminine noun beginning with a vowel sound, French uses son instead of sa for pronunciation:

• son adresse = his/her address

But here the noun is not immediately after the possessive. The word right after the possessive is propre, not adresse. Since propre starts with a consonant sound, there is no awkward vowel clash, so sa is correct:

• sa propre adresse

So:

• son adresse = correct
• sa propre adresse = correct
• son propre adresse = not the normal form here

Yes, it can be left out.

• écrire son adresse = write his/her address
• écrire sa propre adresse = write his/her own address

Propre adds emphasis and makes it especially clear that we mean the sender’s address, not someone else’s. In a sentence about envelopes, that can be useful because both the sender’s and the recipient’s addresses may be relevant.

No. Sa agrees with adresse, not with the sender’s biological sex.

French possessive adjectives agree with the gender and number of the noun possessed:

• son livre = his/her book
• sa lettre = his/her letter
• ses documents = his/her documents

So sa propre adresse does not tell you whether the sender is male or female. It only tells you that adresse is feminine singular.

Yes, expéditeur is the masculine form.

The feminine form is expéditrice.

So you could say:

• L’expéditeur doit aussi écrire sa propre adresse.
• L’expéditrice doit aussi écrire sa propre adresse.

In many general statements, French traditionally uses the masculine form as a generic term, though modern usage may vary depending on style and context.

Yes. A very natural alternative is:

L’expéditeur doit aussi écrire sa propre adresse sur l’enveloppe.

This version keeps the place phrase at the end, which is often the more neutral order.

So the difference is mostly emphasis:

• Sur l’enveloppe, ... = emphasizes where
• ... sur l’enveloppe = more neutral flow

Both mean the same thing.