J’ai oublié mon sac de couchage dans le coffre, alors je retourne à la voiture.

Because French normally uses elision: je becomes j’ before a vowel sound.

• je ai → j’ai
• The same thing happens in:
  • j’aime
  • j’habite
  • j’oublie

So J’ai oublié... is the correct form.

This sentence mixes two time frames:

• J’ai oublié mon sac de couchage... = a completed past action: I forgot / left my sleeping bag...
• alors je retourne à la voiture = what the speaker is doing now as a result: so I’m going back to the car

This is very natural in both French and English: first you mention what happened, then what you do now because of it.

It is the passé composé, a very common French past tense.

It is made with:

• the auxiliary verb avoir in the present: j’ai
• the past participle: oublié

So:

• j’ai oublié = I forgot / I have forgotten

With oublier, French uses avoir, not être.

Because with avoir, the past participle usually does not agree with the subject.

So even though the subject is je, you still say:

• J’ai oublié

not something different for masculine or feminine.

A more advanced point: agreement can happen with avoir if the direct object comes before the verb, but that is not the case here.

Sac de couchage is the normal French expression for sleeping bag.

Literally, it is something like:

• bag of sleeping

French often builds compound nouns this way:

• sac de couchage = sleeping bag
• salle de bain = bathroom
• brosse à dents = toothbrush

So de is just part of the standard noun expression here.

Because mon means my, and sac is a masculine singular noun.

So:

• mon sac = my bag
• ton sac = your bag
• son sac = his/her bag

The important noun is sac, not couchage as a separate item.

Here, le coffre means the trunk of a car.

So:

• dans le coffre = in the trunk

Be careful: coffre can also mean a chest, strongbox, or storage box in other contexts. But with la voiture, it clearly means the car trunk.

For British English learners, this is the part of the car called the boot.

Because the sleeping bag is inside the trunk.

• dans = in / inside

So:

• dans le coffre = in the trunk
• dans la voiture = in the car
• dans le sac = in the bag

If the idea were movement toward something rather than being inside it, French would use a different preposition.

Here alors means so, then, or therefore.

It connects the two ideas:

• I forgot my sleeping bag in the trunk,
• so I’m going back to the car.

It is a very common linking word in spoken and written French.

Retourner usually means to go back or to return to a place.

Revenir usually means to come back.

The difference often depends on point of view:

• je retourne à la voiture = I am going back to the car
• je reviens = I am coming back

In this sentence, the speaker is talking about going back to the car, so retourne is the natural choice.

Because retourner à means to go back to a place.

• à la voiture = back to the car
• dans la voiture = into / inside the car

So the sentence focuses on the destination first: the speaker is heading back to the car. If you wanted to emphasize getting inside it, you might say something like je retourne dans la voiture.

Yes. Retourner can have several meanings depending on context, including:

• go back / return
• turn over
• sometimes turn around

But in je retourne à la voiture, the phrase à la voiture makes the meaning clear: I’m going back to the car.

So this is a good example of a verb whose meaning depends heavily on context.

Yes, but the meaning changes slightly.

• J’ai oublié mon sac de couchage... = I forgot my sleeping bag / I left it behind by forgetting it
• J’ai laissé mon sac de couchage... = I left my sleeping bag there

Oublier emphasizes the fact that it was forgotten.
Laisser emphasizes the fact that it was left somewhere.

In this situation, oublié is more natural if the problem is that you accidentally forgot it.

Because de couchage is not a separate noun phrase here. It is part of the fixed expression sac de couchage.

French often has structures like:

• une table de cuisine
• une brosse à cheveux
• un sac de voyage

So you do not say un sac du couchage here.

It is not absolutely required in every context, but it is very natural and helpful.

The comma marks the break between:

1. what happened
2. the consequence

So it makes the sentence easier to read:

• J’ai oublié mon sac de couchage dans le coffre, alors je retourne à la voiture.

Without the comma, the sentence would still be understandable, but the punctuation helps show the logic clearly.

In everyday speech, speakers often contract and link sounds, so it may sound something like:

• J’ai oublié mon sac d’couchage dans l’coffre, alors j’retourne à la voiture.

That does not mean the written sentence changes. The correct written form remains:

• J’ai oublié mon sac de couchage dans le coffre, alors je retourne à la voiture.

This is just a pronunciation habit in casual spoken French.