J’ai oublié le parasol dans la voiture, alors je retourne vers la dune pour le prendre.

Because the two actions happen at different times:

• J’ai oublié le parasol... = the forgetting happened earlier, so French uses the passé composé.
• ...alors je retourne... = the speaker is going back now, so French uses the present.

So the sentence is basically: I forgot the parasol, so now I’m going back...

If you were telling the whole story completely in the past, you could also say something like je suis retourné instead of je retourne.

It can mean both, and in this sentence it strongly suggests leave behind because you forgot it.

So J’ai oublié le parasol dans la voiture naturally means:

• I forgot the parasol in the car, or
• I left the parasol in the car

French often uses oublier where English might prefer leave behind.

In French, nouns usually need an article much more often than in English.

So French says:

• le parasol
• la voiture
• la dune

where English might simply say parasol, car, or dune in some contexts.

Also, parasol is a masculine singular noun, so the article is le.

Dans means in / inside.

So:

• dans la voiture = inside the car

This is the natural choice when you mean the object is physically in that specific car.

Compare:

• dans la voiture = inside the car
• à la voiture = at/by the car
• en voiture = by car, or in a car in a more general sense

Here, because the parasol is physically inside a particular car, dans la voiture is correct.

Here alors means so, then, or as a result.

It connects the first idea to the consequence:

• I forgot the parasol in the car, so I’m going back...

It is a very common linking word in French.

Retourner means to go back / to return.

Here, je retourne means:

• I’m going back
• I return

It works well because the speaker is moving back toward a place.

A related verb is revenir, which often means to come back. The difference is often about perspective:

• retourner = go back
• revenir = come back

So je retourne vers la dune feels like I’m going back toward the dune.

Vers means toward or in the direction of.

So:

• je retourne vers la dune = I’m going back toward the dune

This does not always emphasize arrival as strongly as à would. It focuses on direction.

Compare:

• vers la dune = toward the dune
• à la dune = to the dune / at the dune

So vers is about movement in that direction.

Pour + infinitive is a very common French pattern meaning in order to... or simply to...

So:

• pour le prendre = to get it / to take it

This explains the purpose of going back:

• I’m going back toward the dune to get it

The verb stays in the infinitive after pour, so prendre is not conjugated.

Le is a direct object pronoun meaning it. It refers back to le parasol.

So:

• le parasol → le
• pour le prendre = to take it

In French, object pronouns usually go before the verb they belong to, even when that verb is an infinitive:

• le prendre = to take it
• la voir = to see her/it
• les chercher = to look for them

So French says le prendre, not prendre le.

Yes. Depending on the exact nuance, French speakers might also say:

• pour aller le chercher = to go get it / to fetch it
• pour le récupérer = to pick it up / retrieve it

The differences are small:

• prendre = take
• aller chercher = go and get
• récupérer = retrieve / pick back up

So pour le prendre is correct, but other versions may sound more natural in some situations.