J’oublie souvent d’apporter de la crème solaire, mais Paul y pense toujours.

Because je shortens to j’ before a vowel sound, and oublie begins with ou-.

So:

• je oublie → incorrect
• j’oublie → correct

This is called elision, and it happens often in French:

• j’aime
• j’habite
• j’écoute

In French, adverbs of frequency like souvent often come after the conjugated verb.

So:

• J’oublie souvent... = I often forget...

This is a very common word order in French. English often puts often before the main verb, but French usually places souvent after it.

Other examples:

• Je mange souvent ici.
• Il parle souvent trop vite.

Because after oublier when you mean to forget to do something, French normally uses de + infinitive.

So:

• oublier de faire quelque chose = to forget to do something

In your sentence:

• J’oublie souvent d’apporter... = I often forget to bring...

Compare:

• J’oublie mon livre. = I forget my book.
• J’oublie d’apporter mon livre. = I forget to bring my book.

So oublier + noun and oublier de + infinitive are two different structures.

Because the verb oublier requires de before another verb in the infinitive.

So the pattern is:

• oublier de + infinitive

Since apporter starts with a vowel, de becomes d’:

• de apporter → d’apporter

This is another example of elision.

Apporter is used for bringing a thing/object.

In this sentence, the thing being brought is:

• de la crème solaire = sunscreen

So apporter is the right choice.

Very roughly:

• apporter = bring something
• emmener / amener = bring/take a person

Examples:

• J’apporte du pain. = I’m bringing bread.
• J’emmène Paul à la plage. = I’m taking Paul to the beach.

Because crème solaire is treated here as an uncountable substance, so French uses a partitive article.

• de la crème solaire = some sunscreen

French often uses partitive articles where English uses no article at all.

Examples:

• Je bois de l’eau. = I drink water.
• Il achète du pain. = He buys bread.
• Elle met de la crème solaire. = She puts on sunscreen.

So even though English just says sunscreen, French says de la crème solaire.

Because la crème solaire would usually mean the sunscreen, referring to a specific sunscreen already identified.

But de la crème solaire means some sunscreen or sunscreen in a general material sense.

Compare:

• J’apporte de la crème solaire. = I’m bringing sunscreen.
• J’apporte la crème solaire. = I’m bringing the sunscreen.

So de la is partitive, while la is definite.

Here, y replaces à cela / à cette chose.

The full idea is:

• Paul pense toujours à apporter de la crème solaire.

To avoid repeating the whole phrase, French uses y:

• Paul y pense toujours.

So here y means something like:

• to it
• about it
• to doing that

In this sentence, it refers to the idea of bringing sunscreen.

Because penser can be followed by à when it means to think about or to remember to think of something.

When a phrase with à refers to a thing or idea, French often replaces it with y.

Pattern:

• penser à quelque chose → y penser

Example:

• Tu penses à ton passeport ? = Are you thinking about your passport?
• Oui, j’y pense. = Yes, I’m thinking about it.

That is exactly what happens in your sentence:

• Paul y pense toujours.

It would sound unnatural in normal French.

French strongly prefers the pronoun y instead of repeating à cela in this kind of sentence.

So:

• Paul y pense toujours. = natural
• Paul pense toujours à cela. = possible, but heavier
• Paul pense à cela toujours. = awkward word order

French usually places y before the conjugated verb:

• il y pense
• nous y pensons
• elle n’y pense pas

In Paul y pense toujours, the adverb toujours comes after the verb phrase, which is a very normal position in French.

So:

• Paul y pense toujours. = Paul always thinks about it.

French adverb placement is flexible in some cases, but this sentence sounds very natural as written.

You may also see:

• Paul pense toujours à apporter de la crème solaire.

Once the object has been replaced by y, the sentence becomes:

• Paul y pense toujours.

They are in the present tense:

• j’oublie = I forget / I am forgetting
• Paul pense = Paul thinks / is thinking

In this sentence, the present tense expresses a habit:

• J’oublie souvent... = I often forget...
• Paul y pense toujours. = Paul always remembers to think about it / always thinks of it

French often uses the present tense for habitual actions, just like English.

Because French, like English, often avoids repetition with pronouns.

Instead of saying:

• Paul pense toujours à apporter de la crème solaire

after the first clause has already mentioned it, French uses y:

• Paul y pense toujours

This makes the sentence more natural and less repetitive.

It is similar to English using it:

• I often forget to bring sunscreen, but Paul always thinks about it.

It means to forget to do something.

So J’oublie souvent d’apporter de la crème solaire means the speaker does not remember to bring sunscreen.

It is not about forgetting information; it is about forgetting an action.

Compare:

• J’oublie son nom. = I forget his name.
• J’oublie de l’appeler. = I forget to call him.

In your sentence, it is clearly the second type: forgetting to do something.