Avant de partir, je plie mon pyjama et j’accroche mon peignoir derrière la porte.

Because after avant, French normally uses de + infinitive when the subject is the same as in the main clause.

So:

• Avant de partir = before leaving / before I leave

This is a very common pattern:

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

If you want a full clause with its own subject, French usually uses avant que + subjunctive:

• Avant qu’il parte = before he leaves

So in your sentence, avant de partir is the normal structure.

French often uses the infinitive where English uses an -ing form.

So:

• Avant de partir literally looks like before to leave
• but it naturally means before leaving

This is a very common difference between the two languages. After many prepositions in French, you use the infinitive:

• sans parler = without speaking
• pour apprendre = in order to learn
• après avoir mangé = after eating / after having eaten

So even though English prefers leaving, French prefers partir.

They are both in the present indicative.

• je plie = I fold
• j’accroche = I hang

In French, the present tense can describe:

• something happening right now
• a habit or routine
• a general truth

In this sentence, it most likely describes a routine or a usual sequence of actions:

• Before leaving, I fold my pajamas and hang my bathrobe behind the door.

So the present tense here works a lot like the English simple present in descriptions of habits.

Because French avoids having je directly before a vowel sound. The e in je is dropped, and you use an apostrophe. This is called elision.

So:

• je accroche becomes j’accroche

The same thing happens with many verbs that begin with a vowel or silent h:

• j’aime
• j’habite
• j’écoute

But you keep je before a consonant:

• je plie
• je parle

French usually uses pyjama as a singular noun to refer to the whole garment or set of sleepwear.

So:

• mon pyjama = my pajamas / my pajama set

This is just one of those vocabulary differences between French and English. English often treats some clothing items as plurals, but French may use a singular form.

So even if the English translation is my pajamas, the French mon pyjama is perfectly normal.

Peignoir usually means a bathrobe, dressing gown, or sometimes a light robe worn over sleepwear.

It is a masculine noun:

• un peignoir
• mon peignoir

Depending on context, English could translate it in slightly different ways, but bathrobe or robe is often the best everyday choice.

No, in normal French you need a determiner before each noun.

So French says:

• mon pyjama
• mon peignoir

and not just:

• mon pyjama et peignoir

This is a major difference from English. French usually repeats articles and possessives more consistently.

So the sentence needs:

• je plie mon pyjama et j’accroche mon peignoir

Because la porte simply means the door, and in context that is usually enough.

French often uses the definite article when the object is obvious from the situation:

• the door of the room
• the nearby door
• the door everyone knows you mean

Using ma porte would sound more like you are emphasizing ownership or contrast:

• my door, not someone else’s

So derrière la porte is the natural everyday phrasing for behind the door.

Derrière is a preposition meaning behind.

It is followed by a noun phrase:

• derrière la porte = behind the door
• derrière la maison = behind the house
• derrière moi = behind me

In your sentence, it tells you where the bathrobe is hung:

• j’accroche mon peignoir derrière la porte = I hang my bathrobe behind the door

So the structure is:

• accrocher quelque chose derrière quelque chose
• to hang something behind something

Yes. With avant de + infinitive, the subject is usually understood to be the same as the subject of the main verb.

Here:

• je plie
• j’accroche
• avant de partir

All of these actions are understood to refer to je.

So the meaning is:

• Before leaving, I fold my pajamas and hang my bathrobe behind the door

If the person leaving were different, French would normally use a full clause instead, often with avant que:

• Avant qu’il parte, je plie mon pyjama.
• Before he leaves, I fold my pajamas.